diff --git a/beacon_node/beacon_chain/src/beacon_chain.rs b/beacon_node/beacon_chain/src/beacon_chain.rs
index 544e6e006eb..1573fa574ff 100644
--- a/beacon_node/beacon_chain/src/beacon_chain.rs
+++ b/beacon_node/beacon_chain/src/beacon_chain.rs
@@ -73,14 +73,14 @@ pub const FORK_CHOICE_DB_KEY: [u8; 32] = [0; 32];
 
 /// The result of a chain segment processing.
 #[derive(Debug)]
-pub enum ChainSegmentResult {
+pub enum ChainSegmentResult<T: EthSpec> {
     /// Processing this chain segment finished successfully.
     Successful { imported_blocks: usize },
     /// There was an error processing this chain segment. Before the error, some blocks could
     /// have been imported.
     Failed {
         imported_blocks: usize,
-        error: BlockError,
+        error: BlockError<T>,
     },
 }
 
@@ -1155,7 +1155,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
     pub fn process_chain_segment(
         &self,
         chain_segment: Vec<SignedBeaconBlock<T::EthSpec>>,
-    ) -> ChainSegmentResult {
+    ) -> ChainSegmentResult<T::EthSpec> {
         let mut filtered_chain_segment = Vec::with_capacity(chain_segment.len());
         let mut imported_blocks = 0;
 
@@ -1293,7 +1293,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
     pub fn verify_block_for_gossip(
         &self,
         block: SignedBeaconBlock<T::EthSpec>,
-    ) -> Result<GossipVerifiedBlock<T>, BlockError> {
+    ) -> Result<GossipVerifiedBlock<T>, BlockError<T::EthSpec>> {
         let slot = block.message.slot;
         let graffiti_string = String::from_utf8(block.message.body.graffiti[..].to_vec())
             .unwrap_or_else(|_| format!("{:?}", &block.message.body.graffiti[..]));
@@ -1339,7 +1339,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
     pub fn process_block<B: IntoFullyVerifiedBlock<T>>(
         &self,
         unverified_block: B,
-    ) -> Result<Hash256, BlockError> {
+    ) -> Result<Hash256, BlockError<T::EthSpec>> {
         // Start the Prometheus timer.
         let full_timer = metrics::start_timer(&metrics::BLOCK_PROCESSING_TIMES);
 
@@ -1350,7 +1350,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
         let block = unverified_block.block().clone();
 
         // A small closure to group the verification and import errors.
-        let import_block = |unverified_block: B| -> Result<Hash256, BlockError> {
+        let import_block = |unverified_block: B| -> Result<Hash256, BlockError<T::EthSpec>> {
             let fully_verified = unverified_block.into_fully_verified_block(self)?;
             self.import_block(fully_verified)
         };
@@ -1423,7 +1423,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
     fn import_block(
         &self,
         fully_verified_block: FullyVerifiedBlock<T>,
-    ) -> Result<Hash256, BlockError> {
+    ) -> Result<Hash256, BlockError<T::EthSpec>> {
         let signed_block = fully_verified_block.block;
         let block = &signed_block.message;
         let block_root = fully_verified_block.block_root;
@@ -2138,8 +2138,8 @@ impl From<BeaconStateError> for Error {
     }
 }
 
-impl ChainSegmentResult {
-    pub fn to_block_error(self) -> Result<(), BlockError> {
+impl<T: EthSpec> ChainSegmentResult<T> {
+    pub fn to_block_error(self) -> Result<(), BlockError<T>> {
         match self {
             ChainSegmentResult::Failed { error, .. } => Err(error),
             ChainSegmentResult::Successful { .. } => Ok(()),
diff --git a/beacon_node/beacon_chain/src/block_verification.rs b/beacon_node/beacon_chain/src/block_verification.rs
index cc929e530b1..42ecc50c619 100644
--- a/beacon_node/beacon_chain/src/block_verification.rs
+++ b/beacon_node/beacon_chain/src/block_verification.rs
@@ -87,9 +87,11 @@ const WRITE_BLOCK_PROCESSING_SSZ: bool = cfg!(feature = "write_ssz_files");
 /// - The block is malformed/invalid (indicated by all results other than `BeaconChainError`.
 /// - We encountered an error whilst trying to verify the block (a `BeaconChainError`).
 #[derive(Debug, PartialEq)]
-pub enum BlockError {
+pub enum BlockError<T: EthSpec> {
     /// The parent block was unknown.
     ParentUnknown(Hash256),
+    /// The parent block was unknown.
+    ParentUnknownCorrect(Box<SignedBeaconBlock<T>>),
     /// The block slot is greater than the present slot.
     FutureSlot {
         present_slot: Slot,
@@ -134,7 +136,7 @@ pub enum BlockError {
     BeaconChainError(BeaconChainError),
 }
 
-impl From<BlockSignatureVerifierError> for BlockError {
+impl<T: EthSpec> From<BlockSignatureVerifierError> for BlockError<T> {
     fn from(e: BlockSignatureVerifierError) -> Self {
         match e {
             // Make a special distinction for `IncorrectBlockProposer` since it indicates an
@@ -151,25 +153,25 @@ impl From<BlockSignatureVerifierError> for BlockError {
     }
 }
 
-impl From<BeaconChainError> for BlockError {
+impl<T: EthSpec> From<BeaconChainError> for BlockError<T> {
     fn from(e: BeaconChainError) -> Self {
         BlockError::BeaconChainError(e)
     }
 }
 
-impl From<BeaconStateError> for BlockError {
+impl<T: EthSpec> From<BeaconStateError> for BlockError<T> {
     fn from(e: BeaconStateError) -> Self {
         BlockError::BeaconChainError(BeaconChainError::BeaconStateError(e))
     }
 }
 
-impl From<SlotProcessingError> for BlockError {
+impl<T: EthSpec> From<SlotProcessingError> for BlockError<T> {
     fn from(e: SlotProcessingError) -> Self {
         BlockError::BeaconChainError(BeaconChainError::SlotProcessingError(e))
     }
 }
 
-impl From<DBError> for BlockError {
+impl<T: EthSpec> From<DBError> for BlockError<T> {
     fn from(e: DBError) -> Self {
         BlockError::BeaconChainError(BeaconChainError::DBError(e))
     }
@@ -188,7 +190,7 @@ impl From<DBError> for BlockError {
 pub fn signature_verify_chain_segment<T: BeaconChainTypes>(
     chain_segment: Vec<(Hash256, SignedBeaconBlock<T::EthSpec>)>,
     chain: &BeaconChain<T>,
-) -> Result<Vec<SignatureVerifiedBlock<T>>, BlockError> {
+) -> Result<Vec<SignatureVerifiedBlock<T>>, BlockError<T::EthSpec>> {
     let (mut parent, slot) = if let Some(block) = chain_segment.first().map(|(_, block)| block) {
         let parent = load_parent(&block.message, chain)?;
         (parent, block.slot())
@@ -278,7 +280,7 @@ pub trait IntoFullyVerifiedBlock<T: BeaconChainTypes> {
     fn into_fully_verified_block(
         self,
         chain: &BeaconChain<T>,
-    ) -> Result<FullyVerifiedBlock<T>, BlockError>;
+    ) -> Result<FullyVerifiedBlock<T>, BlockError<T::EthSpec>>;
 
     fn block(&self) -> &SignedBeaconBlock<T::EthSpec>;
 }
@@ -291,7 +293,7 @@ impl<T: BeaconChainTypes> GossipVerifiedBlock<T> {
     pub fn new(
         block: SignedBeaconBlock<T::EthSpec>,
         chain: &BeaconChain<T>,
-    ) -> Result<Self, BlockError> {
+    ) -> Result<Self, BlockError<T::EthSpec>> {
         // Do not gossip or process blocks from future slots.
         let present_slot_with_tolerance = chain
             .slot_clock
@@ -388,7 +390,7 @@ impl<T: BeaconChainTypes> IntoFullyVerifiedBlock<T> for GossipVerifiedBlock<T> {
     fn into_fully_verified_block(
         self,
         chain: &BeaconChain<T>,
-    ) -> Result<FullyVerifiedBlock<T>, BlockError> {
+    ) -> Result<FullyVerifiedBlock<T>, BlockError<T::EthSpec>> {
         let fully_verified = SignatureVerifiedBlock::from_gossip_verified_block(self, chain)?;
         fully_verified.into_fully_verified_block(chain)
     }
@@ -406,7 +408,7 @@ impl<T: BeaconChainTypes> SignatureVerifiedBlock<T> {
     pub fn new(
         block: SignedBeaconBlock<T::EthSpec>,
         chain: &BeaconChain<T>,
-    ) -> Result<Self, BlockError> {
+    ) -> Result<Self, BlockError<T::EthSpec>> {
         let mut parent = load_parent(&block.message, chain)?;
         let block_root = get_block_root(&block);
 
@@ -438,7 +440,7 @@ impl<T: BeaconChainTypes> SignatureVerifiedBlock<T> {
     pub fn from_gossip_verified_block(
         from: GossipVerifiedBlock<T>,
         chain: &BeaconChain<T>,
-    ) -> Result<Self, BlockError> {
+    ) -> Result<Self, BlockError<T::EthSpec>> {
         let mut parent = from.parent;
         let block = from.block;
 
@@ -471,7 +473,7 @@ impl<T: BeaconChainTypes> IntoFullyVerifiedBlock<T> for SignatureVerifiedBlock<T
     fn into_fully_verified_block(
         self,
         chain: &BeaconChain<T>,
-    ) -> Result<FullyVerifiedBlock<T>, BlockError> {
+    ) -> Result<FullyVerifiedBlock<T>, BlockError<T::EthSpec>> {
         let block = self.block;
         let parent = self
             .parent
@@ -497,7 +499,7 @@ impl<T: BeaconChainTypes> IntoFullyVerifiedBlock<T> for SignedBeaconBlock<T::Eth
     fn into_fully_verified_block(
         self,
         chain: &BeaconChain<T>,
-    ) -> Result<FullyVerifiedBlock<T>, BlockError> {
+    ) -> Result<FullyVerifiedBlock<T>, BlockError<T::EthSpec>> {
         SignatureVerifiedBlock::new(self, chain)?.into_fully_verified_block(chain)
     }
 
@@ -519,7 +521,7 @@ impl<T: BeaconChainTypes> FullyVerifiedBlock<T> {
         block_root: Hash256,
         parent: BeaconSnapshot<T::EthSpec>,
         chain: &BeaconChain<T>,
-    ) -> Result<Self, BlockError> {
+    ) -> Result<Self, BlockError<T::EthSpec>> {
         // Reject any block if its parent is not known to fork choice.
         //
         // A block that is not in fork choice is either:
@@ -670,7 +672,7 @@ impl<T: BeaconChainTypes> FullyVerifiedBlock<T> {
 fn check_block_against_finalized_slot<T: BeaconChainTypes>(
     block: &BeaconBlock<T::EthSpec>,
     chain: &BeaconChain<T>,
-) -> Result<(), BlockError> {
+) -> Result<(), BlockError<T::EthSpec>> {
     let finalized_slot = chain
         .head_info()?
         .finalized_checkpoint
@@ -698,7 +700,7 @@ pub fn check_block_relevancy<T: BeaconChainTypes>(
     signed_block: &SignedBeaconBlock<T::EthSpec>,
     block_root: Option<Hash256>,
     chain: &BeaconChain<T>,
-) -> Result<Hash256, BlockError> {
+) -> Result<Hash256, BlockError<T::EthSpec>> {
     let block = &signed_block.message;
 
     // Do not process blocks from the future.
@@ -754,7 +756,7 @@ pub fn get_block_root<E: EthSpec>(block: &SignedBeaconBlock<E>) -> Hash256 {
 fn load_parent<T: BeaconChainTypes>(
     block: &BeaconBlock<T::EthSpec>,
     chain: &BeaconChain<T>,
-) -> Result<BeaconSnapshot<T::EthSpec>, BlockError> {
+) -> Result<BeaconSnapshot<T::EthSpec>, BlockError<T::EthSpec>> {
     let db_read_timer = metrics::start_timer(&metrics::BLOCK_PROCESSING_DB_READ);
 
     // Reject any block if its parent is not known to fork choice.
@@ -828,12 +830,12 @@ fn load_parent<T: BeaconChainTypes>(
 /// and `Cow::Borrowed(state)` will be returned. Otherwise, the state will be cloned, cheaply
 /// advanced and then returned as a `Cow::Owned`. The end result is that the given `state` is never
 /// mutated to be invalid (in fact, it is never changed beyond a simple committee cache build).
-fn cheap_state_advance_to_obtain_committees<'a, E: EthSpec>(
-    state: &'a mut BeaconState<E>,
+fn cheap_state_advance_to_obtain_committees<'a, T: EthSpec>(
+    state: &'a mut BeaconState<T>,
     block_slot: Slot,
     spec: &ChainSpec,
-) -> Result<Cow<'a, BeaconState<E>>, BlockError> {
-    let block_epoch = block_slot.epoch(E::slots_per_epoch());
+) -> Result<Cow<'a, BeaconState<T>>, BlockError<T>> {
+    let block_epoch = block_slot.epoch(T::slots_per_epoch());
 
     if state.current_epoch() == block_epoch {
         state.build_committee_cache(RelativeEpoch::Current, spec)?;
@@ -864,7 +866,7 @@ fn cheap_state_advance_to_obtain_committees<'a, E: EthSpec>(
 /// Obtains a read-locked `ValidatorPubkeyCache` from the `chain`.
 fn get_validator_pubkey_cache<T: BeaconChainTypes>(
     chain: &BeaconChain<T>,
-) -> Result<RwLockReadGuard<ValidatorPubkeyCache>, BlockError> {
+) -> Result<RwLockReadGuard<ValidatorPubkeyCache>, BlockError<T::EthSpec>> {
     chain
         .validator_pubkey_cache
         .try_read_for(VALIDATOR_PUBKEY_CACHE_LOCK_TIMEOUT)
diff --git a/beacon_node/beacon_chain/src/block_verification/block_processing_outcome.rs b/beacon_node/beacon_chain/src/block_verification/block_processing_outcome.rs
index 8e38af47345..ba29cf149b7 100644
--- a/beacon_node/beacon_chain/src/block_verification/block_processing_outcome.rs
+++ b/beacon_node/beacon_chain/src/block_verification/block_processing_outcome.rs
@@ -1,12 +1,12 @@
 use crate::{BeaconChainError, BlockError};
 use state_processing::BlockProcessingError;
-use types::{Hash256, Slot};
+use types::{EthSpec, Hash256, SignedBeaconBlock, Slot};
 
 /// This is a legacy object that is being kept around to reduce merge conflicts.
 ///
 /// TODO: As soon as this is merged into master, it should be removed as soon as possible.
 #[derive(Debug, PartialEq)]
-pub enum BlockProcessingOutcome {
+pub enum BlockProcessingOutcome<T: EthSpec> {
     /// Block was valid and imported into the block graph.
     Processed {
         block_root: Hash256,
@@ -18,6 +18,7 @@ pub enum BlockProcessingOutcome {
     UnknownValidator(u64),
     /// The parent block was unknown.
     ParentUnknown(Hash256),
+    ParentUnknownCorrect(Box<SignedBeaconBlock<T>>),
     /// The block slot is greater than the present slot.
     FutureSlot {
         present_slot: Slot,
@@ -66,13 +67,16 @@ pub enum BlockProcessingOutcome {
     PerBlockProcessingError(BlockProcessingError),
 }
 
-impl BlockProcessingOutcome {
+impl<T: EthSpec> BlockProcessingOutcome<T> {
     pub fn shim(
-        result: Result<Hash256, BlockError>,
-    ) -> Result<BlockProcessingOutcome, BeaconChainError> {
+        result: Result<Hash256, BlockError<T>>,
+    ) -> Result<BlockProcessingOutcome<T>, BeaconChainError> {
         match result {
             Ok(block_root) => Ok(BlockProcessingOutcome::Processed { block_root }),
             Err(BlockError::ParentUnknown(root)) => Ok(BlockProcessingOutcome::ParentUnknown(root)),
+            Err(BlockError::ParentUnknownCorrect(block)) => {
+                Ok(BlockProcessingOutcome::ParentUnknownCorrect(block))
+            }
             Err(BlockError::FutureSlot {
                 present_slot,
                 block_slot,
diff --git a/beacon_node/network/src/router/processor.rs b/beacon_node/network/src/router/processor.rs
index 76f0cdf449a..929aa6f0e93 100644
--- a/beacon_node/network/src/router/processor.rs
+++ b/beacon_node/network/src/router/processor.rs
@@ -482,7 +482,7 @@ impl<T: BeaconChainTypes> Processor<T> {
         &mut self,
         peer_id: &PeerId,
         block: Box<SignedBeaconBlock<T::EthSpec>>,
-    ) -> Result<GossipVerifiedBlock<T>, BlockError> {
+    ) -> Result<GossipVerifiedBlock<T>, BlockError<T::EthSpec>> {
         let result = self.chain.verify_block_for_gossip(*block.clone());
 
         if let Err(BlockError::ParentUnknown(_)) = result {
diff --git a/beacon_node/network/src/sync/block_processor.rs b/beacon_node/network/src/sync/block_processor.rs
index 3284a38a3e3..eb4f84051e9 100644
--- a/beacon_node/network/src/sync/block_processor.rs
+++ b/beacon_node/network/src/sync/block_processor.rs
@@ -6,7 +6,7 @@ use eth2_libp2p::PeerId;
 use slog::{debug, error, trace, warn};
 use std::sync::{Arc, Weak};
 use tokio::sync::mpsc;
-use types::SignedBeaconBlock;
+use types::{EthSpec, SignedBeaconBlock};
 
 /// Id associated to a block processing request, either a batch or a single block.
 #[derive(Clone, Debug, PartialEq)]
@@ -178,7 +178,10 @@ fn run_fork_choice<T: BeaconChainTypes>(chain: Arc<BeaconChain<T>>, log: &slog::
 }
 
 /// Helper function to handle a `BlockError` from `process_chain_segment`
-fn handle_failed_chain_segment(error: BlockError, log: &slog::Logger) -> Result<(), String> {
+fn handle_failed_chain_segment<T: EthSpec>(
+    error: BlockError<T>,
+    log: &slog::Logger,
+) -> Result<(), String> {
     match error {
         BlockError::ParentUnknown(parent) => {
             // blocks should be sequential and all parents should exist