Though I really doubt existence of this single account with 500k BCT. It seems that 500k BTC is all the money combined from 60k accounts. If true, this would make not rolling trades back an impossibility for mtgox. If true, than mtgox by rolling back simply choses death by a thousand paper cuts as opposite to a quick cyanide injection.
And where are they going to get those bitcoins? MTGOX is done regardless, but these people are not getting the bitcoins that are not theirs.
http://www.cnbc.com/id/37764517/SEC_Offers_Trade_Break_Rules_for_Public_Comment