[Traster]

From wikipedia

>Mt. Gox announced that approximately 850,000 bitcoins belonging to customers and the company were missing and likely stolen, an amount valued at more than $450 million at the time.

This hack pales in comparison to the MT Gox incident. Partly because Bitcoin exchanges are more professional now, have decent security, insurance funds and real commitments. But it's also because this hack is actually relatively small.