I don't see it as a problem anymore. I am not the expert on UBB - far from it. But what may have happened what that the poster originally registered under the bogus name, and created the thread. then they changed it to a legitimate name.
Since UBB (again, as I understand it) caches various pages, the bogus name was still there on the forum's main page. then, when something the page changed, it was re-generated and replaced what was in the cache.
There are also sometimes various race-conditions that can cause strange things to happen, but since this one seems to have cleared itself up, i'm betting on the first case.
There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors
This one isn't even a race condition. On the front page of the saloon, for each topic that appears there, the user name that appears for the most recent post is the user name as it appeared when the post was made. It doesn't bother checking for every user who might have changed their name since then. Generally this doesn't matter much, since the most recent post gets replaced fairly often by a more recent post, as has happened in the time since Satou posted the question.