Mobs will clip through blocks when in a small area with other mobs. Can be reproduced by making a small boxed in area with the corners removed and then spawning a bunch of any mob that fits in it.
What should happen is the mobs should be stuck inside the box and unable to leave due to the walls of it.
What happens is the mobs will randomly clip through the corners and not be able to move and will zip back to their intended location after a short time or when hit by the player