One moment your character is outside a beautiful graphic of a building and the next you are in side a of the walls and it is all lines.
Sometimes you get out and sometimes not.
there are some "holes" in the collision detection in skyrim, which allow you to move "behind" the scene. think of a movie studio lot. a street with buildings. but they're facades, just store fronts. as soon as you go around the back, you see they are fake. its especially common in combat, while jumping, and when there are many characters in close proximity. combat and characters occasionally adjust your location, and apparently don't always do all the required collision checks. same idea while jumping. its the old problem of trying to anticipate every eventuality. you cant walk into a skooma tank, but you can jump into it and get stuck. a robust generic physics system can avoid this. Oblivion was worse. their implementation of havoc physic engine was flawed, resulting in dropped heavy armor occasionally "sinking" though the floor and disappearing, as though you had droppedit on quicksand, instead of a solid surface. in both games, your character will occasionally slip though a crack and start to fall, but the games usually reset you back into the level after a few seconds.
The good side is you see a chest and figure out there is actually a graphical box that is being used as a tool. In my instance it is the inventory of what a vendor character has for sell.
that's done by design. each vendor has a hidden container to track what they have for sale.
I have seen it on purpose as when my son found an online cheat that requires one to hop over a wall where it was not supposed to be possible. He circumnavigates the normal fast travel or front door feature and gone around the games programing sequence. It allows him to walk behind the city where the character drops down through the frame and walk under the structures.
"flying" cheats, as these are called, are specifically not recommended, as they put the game into a state where its behavior is undefined - and therefore may be unstable. this is especially true when you go into an area that usually requires a load screen. you're actually going outside the playable area of the current "doom level" or area. only the buildings and such at the edge of an adjoining level or area are loaded with the current area. No flying is a conscious design choice made by the developers. two main reasons i can think of: 1. much of the game would be too easy with flight or even levitation. think of all that stupid stuff you could skip if you could just fly over it. yet another Falmer! ooh and ahh! 2. loadscreens vs streaming / LOD for loading areas from the hard drive. streaming would most likely be required for flying. altogether a more complex game engine would be required, something more like a flight sim engine with good LOD, rather than a more simplistic shooter engine, which is the technology currently used.