Ever heard of Transaction Malleability Attack?

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This attack seems to have  caused the Bitcoin meltdown.

If this attack is successful then, the attacker can cash in the on bitcoins they never owned.

Bottomline -  About $500 million in bitcoins stored by the exchange have been stolen, comprising 750,000 bitcoins deposited by users of the site, and 100,000 owned by Mt. Gox.

Here is the sad news

In the bankruptcy filing, the exchange (Mt. Gox) reported that it doesn't know what technique -- or techniques -- attackers used to steal the bitcoins, exactly how many were stolen, or when the thefts occurred.

The links below has more details:

http://www.informationweek.com/security/attacks-and-breaches/bitcoin-exchanges-buckle-under-ddos-attacks/d/d-id/1113809


http://www.informationweek.com/security/attacks-and-breaches/mt-gox-bitcoin-meltdown-what-went-wrong/d/d-id/1114091