Shivering Isles, 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).