I've fallen through the world a few times in the Bank of Daggerfall, either at the locked trap door that's in the corner behind the stairs, or where the chest is that's in front of the foot of the stairs where one of the guards likes to stand. In both places I fell through the world and landed in a square patch of ocean, and could see players and NPCs moving around far overhead. I did not have the nerve to try to swim over the side of the square patch of ocean; instead, I just ported to one of my houses.