https://bitcointalk.org/index.php?topic=83794.0#post_toc_21The event's scale was widely disputed; some report a theft of almost 500000 BTC due to related account hacking. However, these reports are sparse and disreputable. Closer inspection puts the losses at closer to 2500 BTC.
They proved they had 500k coins back then. But they were the only big exchange in june 2011. What if they actually had over one million?