I'm not entirely sure how many times this has happened, but it has definitely been more than a few. Now I know how one way to reproduce it is that my champion gets shot with Tristana's ult and I stop flying back on top of a unit. I then proceed to no longer be able to move or attack with my champion. However there have been times when I've been running from a champion and I believe (though not sure) that Zilean cast his speed boost on me, and I could no longer move or attack or even get my champion to respond for several seconds. I wish I could get more on how to reproduce it, but it seems to be random and when another champion casts a spell on me.

Thanks DeusExMachina,

If people can keep posting about when they saw this, we might be able to figure it out more quickly.

I had Tryndamere spin on to me and we got stuck in each other. He then proceeded to kill me

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.