She didn't slip, she misstepped with heels on and her ankle bent. That could seriously hurt someone, and you can tell Britney was really concerned afterwards.
Still, I loved how she said she was alright and pushed through it for the audience! I hope it's feeling much better now, after some rest and TLC.
Side-note: to anyone criticizing this performance, you do realize that all you saw was a 30 second clip in which she was INJURED? Like, what were you expecting when you clicked play? She actually puts on a fantastic show with tons of energy when she isn't in pain.
It IS embarrassing that she lip syncs because she doesn't even bother to use new pre-recorded vocals but gets them straight off the CD.
That being said, I'm glad she's alright. Imagine if this meant her pulling out of her first proper awards show performance in over half a decade. ATRL would implode.