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.