Dont know Chris, I dont have access to vb.org scripts to trace the problem..
But if you want a guess: I would say a hack in showthread.php or getpostbit function in functions.php might be the cause. (If its not a vb bug in the first place) To make sure you have to trace the bug by debugging in the source code.
The problem is server sided though, not related to page visitor, although a visitor related thing is apparently triggering it..