Rabbits (and presumably other entities, but most likely rabbits because they are very small) get pushed into walls and through doors when you exit and re-enter a world.

Steps to Reproduce

1. Make or go inside a house with a door.

2. Fill the house with rabbits until you can't (or barely can) see the floor.

3. Exit and re-enter your world. A handful of rabbits should have escaped out the door, and some may have gotten pushed into the walls and died.