Just thought of my favourite, silly TES trope. I can't remember seeing this one in Morrowind (it's been a while), but it was all over Oblivion and Skyrim. Possibly Fallout 3 too. Anyway, it's a locked, unpickable door near the starting point of a dungeon (or a gate without a lever or something else to that effect). Sometimes it can be something more subtle like a walkway you can't reach, or a big stone somewhere. First thing that pops into my head when I see one of those is always, "oh, I guess that's where I'll come out of once I've cleared this dungeon". Because the devs wouldn't want you having to traverse your steps. Because that would never happen.

Do you have any to add?