It appears a lot of mobs get stuck on ladders in tight spaces.

I have an underground farm where the only exit/entrance is a ladder, and it contains both bees and villagers. Both mobs can go up the ladder, but after that they get stuck on the ladder and cannot descend. I noticed this and did some testing. If a mob can climb a ladder, they are prone to getting stuck if they cannot get off the ladder.

It appears unloading and reloading the chunks (ie, exiting on a server) causes a few mobs to descend temporarily, however if the ladder is tall enough, they could be stuck forever.

Additionally, spiders are prone to this all the time. Spiders always end up stuck on the ceiling in 2x2 spaces with no way out, or along the corners of a house.