The problem with open world games is that you have to define a massive range of objects, including debrie, walls, dungeons, stairs, it’s not like a level in an FPS where you’re mostly railroaded into everything within a contained environment. All open world games are notorious for their bugs, especially collision detection bugs, but Bethesda have gone out of their way to iron those out for Skyrim.
Pete Hines said in an interview “It’s something we continue to try to address and design for, if you go back and look, Fallout 3 was an incredibly stable game. Certainly not bug-free, but there’s a difference for us between a rock that’s floating a little above the ground, which is technically a bug, and one you might have that causes your game to crash or your save-games to get corrupted. So there’s degrees. We start at the top and work our way down. Does the game load when you click on it? Does it save properly? That stuff. So it’s something we’re cognisant of. I think for Skyrim we built a number of things into the game to cover that and to try to improve that.”
Bugs are common in these games because having to define an entire engine and a world with a lack of invisible walls along with an attitude of “Nah, they wont manage that.” causes major problems. To cite an example, Grand Theft Auto 3 had multiple buildings that were completely hollow, and you could fly through them with the Dodo. There were also places you could fly under the ocean into Blue Hell. This was significantly cleaned up in Vice City and all but removed in San Andreas.
It’s very unlikely a game that’s the scope of Skyrim along with its depth will be free of bugs, especially in the AI which is often thick as mince and exploitable to a ridiculous extent. Most bugs don’t appear if you play the game as intended but that’s not anyone’s idea of fun.