Another heartbreaking (but no less attempt-discouraging) bug that would occasionally happen would be that after loading a saved game you'd discover that the contents of your bags had all turned into UW's ubiquitous piles debris (or pools of blood, I can't remember anymore), and that your saved game had been saved in a corrupted state.
Underworld wasn't the only Ultima that had a hit-it-with-a-stick-till-it-does-something-funny-or-undefined quality to it. You could do some fairly unsanctioned things (climing on top of stuff, going where you weren't supposed to go) in 7, and in Ultima 9 (Ascension) you could visit places long before you were "meant" by mountain climbing with stairways constructed from objects out of your inventory (my favourite was using books and loaves of bread).
Origin games always had such a rough-but-polished quality to them. Whenever I broke an Ultima in this manner it never quite shattered my suspension of disbelief (like, for example falling out of a Quake level into empty space) but instead gave me the impression that wow, they had a fairly open environment here where you could expect the unexpected.
With a lot of modern games that seems to have been lost, where typical map design is simply a straight march through a series of set pieces, with invisible walls blocking off any chance of looking behind or underneath something or coaxing some kind of unsanctioned behaviour out of the game. Not to mention the flood of quality control these games enjoy..
Too bad! On the other hand, one of the most popular games out right now, Minecraft, seems to have brought back a little bit of this spirit.
[EDIT: did any other UW players here hoard objects in that room near the dwarves on level 2? Good times.. ]