Despite an unhealthy obsession with movie goofs, I somehow missed this all these years. It seems that at the end of the first Teen Wolf (the one starring Michael J. Fox), an extra standing in the bleachers revealed his stuff for a second and nobody noticed so it was in the movie:
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This is definitely a better goof than the storm trooper who hit his head in Star Wars. Also, it's nowhere to be found on Teen Wolf's IMDB goofs page, though this is:
Priorities, people!
