If you're playing Oblivion's Expansion Pack, Shivering Isles, you have a timebomb of an issue on your hands. Yeah, it's obviously something that Bethesda will be fixing quickly (and has already been patched for PC users thanks to some quick mod work by the players), but if you're playing the game like mad on the 360, you better stop before you completely break your save game:
More details than you'll ever want to know (no spoilers) over here
, including mods that'll fix the bug in the PC version and even attempt to fix your savegame if you've already had the bug.
What exactly happens? Here's a short summary:
The basis of bug is that an internal "space" of identification numbers for objects created in the game (e.g., a dropped arrow, a new creature, etc.) becomes exhausted -- at which point the newly created objects disappear from the game world. (Odd behavior of merchants has also been reported, and additional problems are likely.)
The immediate cause of the exhaustion is scripts for six NPCs in SI which are erroneously using up several identification numbers per frame of gameplay (hence higher FPS systems get hit sooner). The PC patch works by correcting the scripts for these NPCs.
Roughly translated - once the bug hits you, no new creatures, weapons, items, armor, anything, appear in the game world.
And a forum post
over on Bethesda's site if you're interested in more reading.
Keep this in mind though - nobody has fully confirmed that this will affect 360 users (although it is assumed it will). It DOES affect PC users though:
It is still uknown whether the game will roll those IDs back to FF or to 00 on the XBOX. Judging by this test, it SHOULD effect XBOX users exactly the same way it does PC players! IE: LOSS OF ITEMS LEFT IN THE GAME WORLD! However, there can be no definate answer until someone hits 80+ hours with SI and reports a loss of items.
In short - if you're running the PC version, get a mod ASAP from the Wiki page up top to fix it. If you're playing the 360 version, you might want to stop playing for a little bit and let Bethesda fix the issue (or at least come out and say that it won't affect 360 users).