Cassiopeia bug
I just played a game where my champion was facing away from Cass when she ulted, but I was stunned anyway until she killed me which was about 5 seconds late. I know I was facing away since the animation showed me facing away from her while I was frozen. Plus the fact that I was perma stunned until I died.
I had just been stunned a few seconds before by another champ, so that may have something to do with it.