Audio problem: When D-Day, Bluto and Flounder are running out of Wormer's office after the horse died, there is a sound effect of them screaming as they flee the scene. This exact same sound effect is used when the boys run out of the all-black bar, except Bluto and D-Day weren't there; it was Flounder, Kroger and Boon. (00:30:40 - 01:17:15)
Audio problem: At the bar after the tall black man asks to dance with the boys' dates, he lifts the booth table up off the floor. There's a sound like it's being forcibly ripped out of the floor, yet the table has a round bottom with no visible bolts or screws. While it may have been heavy, it wasn't secured to the floor.
Audio problem: When Bluto is using the ladder to look in the sorority house, the ladder gives a clanging sound like an aluminum extension ladder. The ladder Bluto is using is a single frame wooden ladder which wouldn't make any clanging sounds at all.
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