Ugh, poor Britney. Usually it's Brit'sknees (gerrit?!) that are her issue, but here she is, letting loose at the end of her Vegas show - after being held aloft, spun around, and generally manhandled - and she promptly goes over on her ankle. Typical. It's at the end of the vid in case you're in a hurry. 

In an attempt to alleviate concerned minds, she took to twitter to say she's A-OK.
 

According to BBC News, Britney "was forced to perform most of the rest of the song from a sitting position, before being helped to her feet by her dancers." She has been ordered by doctors to cancel her shows for the next two days.