Minions in the Graveyard can be affected by targeted Battlecries, Spells and use their Battlecry

[Patashu]

1.) Bane of Doom and Mortal Coil targeting a minion that dies before the spell goes off may or may not trigger the side-effect, seemingly inconsistently.

[Hearthstone Science] Bane of Doom, Mortal Coil and the Graveyard

Technical explanation:

In Hearthstone, Spells and Battlecries targeting a minion still activate and affect that minion if the minion is killed in an earlier Phase, such as by Violet Teacher spawning a token and a friendly Knife Juggler reducing it to 0 Health. Furthermore, when the minion moves to the graveyard, its attributes, such as Health, are reset, and all attached Enchantments are removed.

If Bane of Doom or Mortal Coil are used to target such a minion, they damage the minion after it has been moved to the graveyard. This means their side-effect will go off if they reduce the reset version of the minion to 0 or less Health, regardless of their state prior to dying.

2.) Battlecries that target a minion that is already in the Graveyard will still resolve and have their 'expected effect'. For example, playing a Youthful Brewmaster on Illidan Stormrage, if Illidan Stormrage dies before the Battlecry Phase, will return Illidan Stormrage from the Graveyard to the hand.

[Hearthstone Science] Resurrect Minion no longer in the Graveyard (Death Events vs Graveyard)

The following has been fixed in Patch 7.1.0.17720 (see #58):

If you play a minion and it dies before the Secret Activation Phase, and your opponent has Mirror Entity in play, it will still create a copy of the minion.

Weird Dread Infernal interaction with Explosive Sheep and Mirror Entity｜恐怖的煉獄火 vs. 123自爆綿羊&鏡像體

[Patashu]

This bug overlaps with #58

[VoidMayonnaise]

Now minions in graveyard (Bronzebeard & Blademaster) cannot trigger Mirror Entity anymore. Fixed in patch 7.1.