The Elder Scrolls IV: Shivering Isles Bug Documented

The UESPWiki has documented a nasty bug discovered in Bethesda's The Elder Scrolls IV: Shivering Isles expansion pack.
A very serious bug has been found with Shivering Isles that will render the game unplayable in many ways about 50 to 120 hours of play (better FPS means bug hits earlier) after installing the expansion. The bug will occur if you have SI installed -- regardless of whether you access SI content or not.

The bug is thoroughly confirmed on the PC, and there are now reports of it affecting Xbox users too, although it takes more hours of play to affect Xbox users.

While the situation appeared quite bad initially, a patch mod is now available for the PC and a tool is available to repair savegames that are affected by the bug or that are likely hit by it in the near future. Moreover, there is an official response from Bethesda:

Regarding the issue in which Form ID's are being used at a high rate in the Shivering Isles content; we are aware of the issue and we are currently looking into a solution. We appreciate your patience, especially from those of you affected by this issue, as we carefully work out a fix that will correct this problem without adding any new issues. --NothingCatchy, Developer

A patch mod has been released to correct the error, so pay a visit to the UESPWiki if you're looking for a place to download it.