There was a big security flaw with blockchain.info in which they reused R values, which essentially allows an attacker to calculate the private key from the public key. Several users had their coins stolen, and so the user johoe who first detected the flaw emptied the other wallets which were at risk, to the tune of 870 BTC, and returned the coins back to blockchain.info to be redistributed to the rightful owners.

You can read more in this thread: https://bitcointalk.org/index.php?topic=581411.0