, the first and apparently only Oblivion expansion, was released some time ago. Not so long after that, a rather serious bug was found in the game. You can read about the bug on the Unofficial Elder Scrolls Pages
. A user-made fix was posted on the official forums shortly after the bug's cause was determined.
Also, a beta patch to fix the bug has been released for the PC version of the game, which you can get here
. The final patch, for both the XBox 360 and PC version of the game, is supposed to be released on Monday (30th April)