Cats try to jump onto carpets as if they were full blocks. Iron golems can sometimes jump around "on" a button and mobs will jump around spinning if you place a block and they land on that block. Fish spin around in seagrass and kelp and so do dolphins.

How to reproduce:
Try any of the scenarios I said above

What I expected:
The mobs will just move normally

What I saw:
The mobs behaved weirdly such as jumping around "on" blocks that aren't full blocks.