Continuity mistake: When the "Deathmobile" first appears, it spins around 180° and is parked parallel to the curb in front of the Beauty Bar (just past the grandstand) when Bluto pops out dressed as a pirate. When seen next it's now at a 45° angle facing the grandstand in front of the Fashion Store, but it was never heard or seen moving between shots. (01:39:55)
Continuity mistake: At the end of the parade, Neidermeyer has a rifle in his hand as Flounder approaches him with a seltzer bottle calling his name. Neidermeyer lifts the flap of his pocket to reveal one round for his weapon. He loads it in to the chamber and fires. He then reaches in to his pocket a second time, loads and fires, but the first shot showed only one round in the pocket.
Continuity mistake: While the chaos during the parade scene near the end of the movie is happening, look at the store windows during some of the shots of the people running up and down the streets. Some of the light up signs and other posted items on the windows, appear to be reversed as if the same shots were used several times, just reversed time from time to make it appear that people were running in both directions of the street.
Continuity mistake: At the start of the party, the character collecting the coats throws them on the floor in front of the door. Yet when Kent Dorfman and his date enters the house, they are obviously not there, as the door opens easily and they do not have to step over them when they enter.
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