The opponent has an Imp Gang Boss on the board. I use the Arcane Craftiness hero power. One missile hit the Imp Gang Boss, it triggered and spawned an Imp, the second missile hit the summoned Imp and killed it, but the hero power stopped working (no additional missiles as intended).
When you first hit the dragon egg with arcane craftiness, the whelp is generated. If the second missile hits the whelp, it dies but the missiles do not recast.
Arcane Craftiness: "Fire two missiles that deal 1 damage each. If any minion dies, repeat this."
https://us.forums.blizzard.com/en/hearthstone/t/tot-3-bugs-in-heroic-mode/14241
https://us.forums.blizzard.com/en/hearthstone/t/arcane-craftiness-and-dragon-egg/16494
https://us.forums.blizzard.com/en/hearthstone/t/tomb-of-terror-arcane-craftiness/14467