Seems as though they've discovered a bad bug in the expansion to Oblivion, Shivering Isles on the PC version (only). Object IDs weren't being freed up after use, causing them to run out of slots and stopping new objects being created, causing a crash (and breaking of that save game I guess).
How quickly it happens depends on framerate - the faster the rendering the more quickly you fill up the IDs. Apparently it can take from 50 hours gameplay onwards, though maybe Hexus users with super rigs might notice it quicker.
A beta patch (hotfix) that fixes this problem can be found at the elderscrolls website.