Serious Shivering Isles Ref Bug -- April 7th, 2007
A very serious bug has been found with Shivering Isles that will render the game unplayable after about 50 to 120 hours of play (higher performance systems will be hit sooner). The bug will occur regardless of whether you access SI content or not. 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. (CTDs and other problems are also almost certain.)
A patch is available which PC users should install immediately! If you wait until the bug shows up, then not only is your savegame unfixable, but most savegames prior to the one with the bug are highly subject to developing the problem later.
It is not known whether this affects XBox users as well (it is reasonable to assume that it would, but might not hit until around 150 hours of play assuming 30 FPS on XBox).
Shouldn't be a problem for PlayStation since Shivering Isles is not yet available for PS3.
For more info, see Reference Bug.

Credit to UESP.net