When a mob is pursuing a mob or player, the mob sometimes doesn't path find around any holes or lava in the way, and ends up falling into it.

This is a repeatable test. Put a villager on one side of a box, and a zombie on the other, with lava separating the middle in such a way, that it's the only path for the zombie to go to get to the villager. The zombie ends up going straight towards the villager and falls directly into the lava/hole.

Difficulty: Easy

Gamerule: doMobSpawning False