I doubt that security system has failed, users weren't responsible for their funds enough, same thing could happen with your internet banking.
I really hope that it is the case. Blockchain.info is (probably) the most popular online service right now, far ahead of other service providers such as Coinbase and Armory. Any failure will be catastrophic for the Bitcoin.
I guess they would already notice it and halt the transacions...there would be way more topics claiming they got their coins stollen cuz of glitch.