I have a good friend who is LDS and also an ND graduate. While he is fightin' irish through and through, his kids are BYU fans. He happened to be on campus with his kids back when the Rudy movie was being filmed and got to be extras as part of the crowd. There was one scene that the director apparently wanted on the film. It was one where the ND band leads the team out (of the tunnel perhaps?). Anyway, they only had one shot at it with the schedule, band, and lighting. It was all set, but the director and film crew failed to notice my friend's son, the only one in the crowd wearing a big white and blue lettered BYU sweatshirt make his way to the front of the rope. They shot the scene and only at the end did the director notice the problem. What followed was a long list of expletives. They couldn't redo the scene for some reason... and that, my friends, is why the ND band/team tradition is not in the Rudy movie. All due to one young man, wearing his BYU colors.
And yes, this is a true story.