I hope this isn't a duplicate...at least I couldn't find it. But there are some annoying walking bugs that have been driving me crazy, this is the only one I've been able to reproduce in a clean world so far:

So take a 2-block high garden (less room for jumping or endermen) with a three-block high opening. When attempting to walk over a lilypad under the two-block high ceiling while under a 3-block high ceiling, you can't move.

I've attached a world save you can see it in action in.