Spencergaum found the secrets:
The reason why Britney Spears ' performance wasn't so good: her shoe BROKE! Alot of people have sent me messages that I have been inconsistent and switching between her feet. Here is my response: It is HER right foot, but when she's facing the camera, it looks to US (the audience) like her left foot, because it's a mirror image. But when she's facing the back of the stage, it's no longer reversed, and our right is her right, just as our left is her left. So it's actually her RIGHT foot the whole time, but from OUR PERSPECTIVE, it's her left.
Spencergaum is not the only person defending Britney Spears' VMA performance, make sure check Chris Crocker's (MySpace page) silly Youtube video Leave Britney Alove.
Source: Updated! Britney Spears VMA Truth @ Youtube