Corrected entry: When William arrives at the concert, you can hear Black Sabbath playing. He then spends some time backstage prior to the support band going on stage. In reality, a support band would always play before the headline act.
pinkwafer
3rd Oct 2005
Correction: Actually, this is just background music, similar to most of the other songs used in the movie. The actual characters can't hear it. If it were meant to be Black Sabbath performing, they would have used a live recording of "Paranoid" instead of the studio version. Plus, Black Sabbath doesn't even arrive at the concert until about fifteen minutes later.