After the floor show when Riff and Magenta burst in and do their thing. As the cast clears the stage for Frank's final torch song, Janet steps on Brad's foot. Watch for the expression of pain. You have to be quick though, it is a split second thing.
I saw in a documentary about RHPS, that Janet stepped on his foot for revenge on when he slammed his fist down on her hand in the dinner scene when he says, "Just what are you implying?"
That slip up was told by Brad in the collector's edition of Rocky Horror. And I think it is quite a realistic slip up, because no doubt it probably would have happened if it was a real story, I mean, they are all trying to walk backwards in heels!