I suggested it sounded like a race condition several days ago in
https://bitcointalk.org/index.php?topic=429769.msg4738455#msg4738455 - this was dropped into #mtgox and also sent to MagicalTux, but nobody got in touch.
Obviously we have no idea how their code is working though, and i would think that they know about race conditions and how to fix them anyway, because their trade matching engine needs to have locking.
Ah, at least someone else observing the same thing with the same conclusion! Thanks for notifying! I'm pretty convinced it has to be a race condition in their code and it is most likely the SelectCoins() function that is not properly guarded.