diff --git a/lib/bundler/installer/parallel_installer.rb b/lib/bundler/installer/parallel_installer.rb
index c973233d99d..70939ceeb70 100644
--- a/lib/bundler/installer/parallel_installer.rb
+++ b/lib/bundler/installer/parallel_installer.rb
@@ -47,23 +47,22 @@ def ignorable_dependency?(dep)
       # sure needed dependencies have been installed.
       def dependencies_installed?(all_specs)
         installed_specs = all_specs.select(&:installed?).map(&:name)
-        dependencies(all_specs.map(&:name)).all? {|d| installed_specs.include? d.name }
+        dependencies.all? {|d| installed_specs.include? d.name }
       end
 
       # Represents only the non-development dependencies, the ones that are
       # itself and are in the total list.
-      def dependencies(all_spec_names)
+      def dependencies
         @dependencies ||= begin
-          deps = all_dependencies.reject {|dep| ignorable_dependency? dep }
-          missing = deps.reject {|dep| all_spec_names.include? dep.name }
-          unless missing.empty?
-            raise Bundler::LockfileError, "Your Gemfile.lock is corrupt. The following #{missing.size > 1 ? "gems are" : "gem is"} missing " \
-                                "from the DEPENDENCIES section: '#{missing.map(&:name).join('\' \'')}'"
-          end
-          deps
+          all_dependencies.reject {|dep| ignorable_dependency? dep }
         end
       end
 
+      def missing_lockfile_dependencies(all_spec_names)
+        deps = all_dependencies.reject {|dep| ignorable_dependency? dep }
+        deps.reject {|dep| all_spec_names.include? dep.name }
+      end
+
       # Represents all dependencies
       def all_dependencies
         @spec.dependencies
@@ -79,6 +78,8 @@ def self.max_threads
       [Bundler.settings[:jobs].to_i - 1, 1].max
     end
 
+    attr_reader :size
+
     def initialize(installer, all_specs, size, standalone, force)
       @installer = installer
       @size = size
@@ -92,6 +93,7 @@ def call
       # TODO:  remove in bundler 2.0
       require "bundler/gem_remote_fetcher" if RUBY_VERSION < "1.9"
 
+      check_for_corrupt_lockfile
       enqueue_specs
       process_specs until @specs.all?(&:installed?) || @specs.any?(&:failed?)
       handle_error if @specs.any?(&:failed?)
@@ -135,6 +137,31 @@ def handle_error
       raise Bundler::InstallError, errors.map(&:to_s).join("\n\n")
     end
 
+    def check_for_corrupt_lockfile
+      missing_dependencies = @specs.map do |s|
+        [
+          s,
+          s.missing_lockfile_dependencies(@specs.map(&:name)),
+        ]
+      end.reject { |a| a.last.empty? }
+      return if missing_dependencies.empty?
+
+      warning = []
+      warning << "Your lockfile was created by an old Bundler that left some things out."
+      if @size != 1
+        warning << "Because of the missing DEPENDENCIES, we can only install gems one at a time, instead of installing #{@size} at a time."
+        @size = 1
+      end
+      warning << "You can fix this by adding the missing gems to your Gemfile, running bundle install, and then removing the gems from your Gemfile."
+      warning << "The missing gems are:"
+
+      missing_dependencies.each do |spec, missing|
+        warning << "* #{missing.map(&:name).join(", ")} depended upon by #{spec.name}"
+      end
+
+      Bundler.ui.warn(warning.join("\n"))
+    end
+
     # Keys in the remains hash represent uninstalled gems specs.
     # We enqueue all gem specs that do not have any dependencies.
     # Later we call this lambda again to install specs that depended on
diff --git a/spec/bundler/installer/parallel_installer_spec.rb b/spec/bundler/installer/parallel_installer_spec.rb
new file mode 100644
index 00000000000..0f4e7dba9ab
--- /dev/null
+++ b/spec/bundler/installer/parallel_installer_spec.rb
@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+require "spec_helper"
+require "bundler/installer/parallel_installer"
+
+describe Bundler::ParallelInstaller do
+  let(:installer) { instance_double("Installer") }
+  let(:all_specs) { [] }
+  let(:size) { 1 }
+  let(:standalone) { false }
+  let(:force) { false }
+
+  subject { described_class.new(installer, all_specs, size, standalone, force) }
+
+  context "when dependencies that are not on the overall installation list are the only ones not installed" do
+    let(:all_specs) do
+      [
+        build_spec("alpha", "1.0") {|s| s.runtime "a", "1" },
+      ].flatten
+    end
+
+    it "prints a warning" do
+      expect(Bundler.ui).to receive(:warn).with(<<-W.strip)
+Your lockfile was created by an old Bundler that left some things out.
+You can fix this by adding the missing gems to your Gemfile, running bundle install, and then removing the gems from your Gemfile.
+The missing gems are:
+* a depended upon by alpha
+      W
+      subject.check_for_corrupt_lockfile
+    end
+
+    context "when size > 1" do
+      let(:size) { 500 }
+
+      it "prints a warning and sets size to 1" do
+        expect(Bundler.ui).to receive(:warn).with(<<-W.strip)
+Your lockfile was created by an old Bundler that left some things out.
+Because of the missing DEPENDENCIES, we can only install gems one at a time, instead of installing 500 at a time.
+You can fix this by adding the missing gems to your Gemfile, running bundle install, and then removing the gems from your Gemfile.
+The missing gems are:
+* a depended upon by alpha
+        W
+        subject.check_for_corrupt_lockfile
+        expect(subject.size).to eq(1)
+      end
+    end
+  end
+end
diff --git a/spec/install/parallel/spec_installation_spec.rb b/spec/bundler/installer/spec_installation_spec.rb
similarity index 67%
rename from spec/install/parallel/spec_installation_spec.rb
rename to spec/bundler/installer/spec_installation_spec.rb
index 7bbd2bd0e69..b18695bf268 100644
--- a/spec/install/parallel/spec_installation_spec.rb
+++ b/spec/bundler/installer/spec_installation_spec.rb
@@ -58,20 +58,5 @@ def a_spec.name
         expect(spec.dependencies_installed?(all_specs)).to be_falsey
       end
     end
-
-    context "when dependencies that are not on the overall installation list are the only ones not installed" do
-      it "raises an error" do
-        dependencies = []
-        dependencies << instance_double("SpecInstallation", :spec => "alpha", :name => "alpha", :installed? => true, :all_dependencies => [], :type => :production)
-        all_specs = dependencies + [instance_double("SpecInstallation", :spec => "gamma", :name => "gamma", :installed? => false, :all_dependencies => [], :type => :production)]
-        # Add dependency which is not in all_specs
-        dependencies << instance_double("SpecInstallation", :spec => "beta", :name => "beta", :installed? => false, :all_dependencies => [], :type => :production)
-        dependencies << instance_double("SpecInstallation", :spec => "delta", :name => "delta", :installed? => false, :all_dependencies => [], :type => :production)
-        spec = described_class.new(dep)
-        allow(spec).to receive(:all_dependencies).and_return(dependencies)
-        expect { spec.dependencies_installed?(all_specs) }.
-          to raise_error(Bundler::LockfileError, /Your Gemfile.lock is corrupt\. The following.*'beta' 'delta'/)
-      end
-    end
   end
 end