Both the PC and 360 versions of the Oblivion expansion The Shivering Isles seem to have come complete with a game-crippling bug that causes objects to disappear from the game world. Stuff from your inventory, in your house, whatever, it all starts to go poof. The bug starts buggin' once the Shivering Isles expansion is installed, whether you actually access any of the new content or not, but it'll take it a around 50-120 hours to start mucking up your gameplay. You can read some first-hand descriptions of the problem in the Shivering Isles forums.
Oblivion fans have developed an unofficial mod that fixes the PC bug, but 360 owners will have to wait for Bethesda to sort it out for them. Bethesda's aware of the problem, and working on it, but the bottom line is, for now, the only way to protect yourself is to not download the expansion. If you already have it, you might want stop playing until further notice.
