I remember seeing something like this when everyone was trying to figure out who JP sold AO to. It feels good to finally have an idea how this sort of stuff came about. My guess is that it has something to do with that page that says "Invalid Thread Specified..., etc." That page is supposed to come up, and since it didn't in this case there is something in the database that keeps track of this that isn't updated correctly, thus trying to display it instead of showing the "Invalid Thread" message. It probably isn't wise to just "patch this up" by checking for replies before displaying it, but should rather going to the core of the problem and make sure the code and database reflects this so it doesn't happen again?

Chances are this could acturally be an issue with vBulletin, since I *think* that is a standard part of the message board. Plus it has been a rarely seen issue since at least about 6 months ago. Probably at some point in time some code got swapped out and forgotten about, so this check is not done? Just tossing out a wild guess as to the cause...