A security vulnerability was found by hackers during an update to Blockchain.info's wallet service. Luckily, these hackers, who stole hundreds of bitcoins, returned them promptly after the issue was fixed. Supposedly the hackers only took the bitcoins to keep them safe from other, less ethical hackers.
Hundreds of bitcoins were stolen from Blockchain.info users. The company admitted that a technical issue during a software update of the popular Bitcoin wallet created a temporary security hole, and is now reimbursing the users who lost their bitcoins.