Mobs will not jump over a single of blocks, even if they are lured or running away. They tend to circle on themselves instead.
This does not affect spiders, cave spiders, zombie pigmen, wolves, silverfish and endermen. Flying mobs are unaffected.

There are several ways to reproduce this but the easiest way is with a hostile mob in creative.

1. Surround the mob with 1 layer of blocks.
2. Switch to survival mode.
Notice that the mob will continuously turn.

A combination of mobs can be used to test this bug (put mobs in different cells but keep them within range):
1. Zombie and villager separated and surrounded by blocks.
2. Creeper and ocelot.
3. Animals and player holding animal feed(carrots for pigs,etc.)
4. Iron golem and hostile mobs.
5. Ocelot and player.