I've ran in to a number of situations where I got stuck. Once when uses Yi's Alpha Strike on a target that either went invis or teleported, don't remember which. I just disappeared from the screen entirely. I was stuck in the position where I would have landed on the person.
Another time was with Evelynn, I was stealthed and went to attack an enemy champion, I walked through some enemy creeps to do it. Once the attack went off and I unstealthed, I was stuck inside the enemy creeps.
I've also gotten stuck inside Cryophoenix's ice wall when it was cast directly on top of me. I don't believe that is the purpose of this skill?
I've also seen other players get stuck inside turrets and inhibitors.