In the 'High School Vocal Competition' where Oz runs from his Lacrosse team to sing with Heather, we see a very different and very MIDDLE AGED choir singing with them on stage.
The two girls with one line each at the start when Oz first sings with the choir are no where to be seen.
I read somewere the director's explanation for the choir girls mistake: she was not in town for the other shots and it was too expensive to wait for her (he only mentioned one girl missing...). He didn't expect people to notice that. Tough luck! ;-)
If you remember the guy that shouts M. I. L. F. in the beginning, he is also in the choir. it's not really a slip up but just a hidden bit which I saw.
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