Visible crew/equipment: In the scene just prior to the food fight, as John Belushi goes through the cafeteria line putting the food on his tray (which moves frequently), note the cameraman's feet in the mirror over the food following him. (00:33:30)
Visible crew/equipment: Near the end of the movie during the parade riot and Bluto is trying to escape, he jumps up on top of a building. You can see one of the uniformed soldiers bending down to give him help up onto the awning. (01:40:05)
Visible crew/equipment: Look at the blank wall just in front of D-Day when he rides his motorcycle up the stairs of the frat house - it has the most obvious boom shadow you will ever see in a film. (00:09:15)
Visible crew/equipment: During the scene where Otter is hitting golf balls at the ROTC cadets, Niedermayer is wearing a highly polished chrome helmet. In at least one shot, you can see a movie light reflected in it. (00:20:10)
Answer: Sometimes these things get left in because it's simply the best take. (The child covering his ears before the gunshot in "North by Northwest," for example.) It could also be that John Landis cast the extra because he wanted someone with goofy expressions in the crowd. He simply could have told the extras "Ok, be excited that you're at a parade," and that's how this extra did it.
Captain Defenestrator