There was a bug in the implementation of Bitcoin-QT 0.7.2 (the "reference" implementation of the protocol.)  Well, not a bug, so much as an oversight in the configuration of the underlying database.

It caused a problem (a hard fork in the blockchain) which was solved by the majority of miners temporarily downgrading to 0.7.2.

Hence the "News" link in purple at the top of your screen, inviting everyone to upgrade to 0.8.1 or better before May 15, as the limitations of 0.7.2 will be ignored after that date.  (http://bitcoin.org/may15.html)

It's important to note that the bug wasn't in the Bitcoin protocol, just in a certain implementation.