From Yahoo! News:

Scientists Retract Story on Ecstasy Brain Damage

WASHINGTON (Reuters) - Researchers horrified to find they had used a mislabeled bottle in an experiment retracted their findings on Friday, saying they had failed to show the drug Ecstasy can cause a certain pattern of brain damage. [Yahoo! News]

Somewhat luckily, the tests were done on monkeys and not humans. But still, that's a pretty big goof. It makes you wonder if there are any current medications on the market that had similar things happen.