A Wall Glitch

 See this pot/wall? Looks normal, huh? Well, when you step closer...

 Suddenly the whole rock is gone and you're seeing this weird endless-space thing.

I'm pretty sure that this was in the Labyrinthian during the College of Winterhold Quest, The Staff of Magnus. After the skeletal dragon, it's in the first alcove on the left. It's like, a dip in the rock wall with a urn in it. I walked into the rock and saw that. Weird.