When I become a fox in the Be A Mob pack, there is an issue with 2 triggers.
Sleeping resets the ability to crawl through one-block high spaces.
Someone else unmorphing disables that ability too sometimes.
So if I crawl into my fox den, sleep, then I won't be able to crawl out of my den.

1 Become a fox and crawl through a one block tunnel into a 2 block high cavern
2 Sleep in the cavern
3 Try to crawl out of the 1 block hole you came through

Expected Results: You will slip through the tunnel.
Observed Results: You are unable to go back into the hole.

I think this happens with other one block high mobs, I will have to test. I will comment my results.