Revealing mistake: After the Deathmobile ramming, when all the grandstand participants are in the heap of broken two by fours, you can see several chainsaw gashes in the structure made by the crew in order to pre-weaken the grandstand and facilitate its collapse.
Revealing mistake: When the people from the grandstand are flying up in the air after the Deathmobile hits it, you can tell they used a trampoline to get the effect as the woman are in their stocking feet; it would have been a safety issue if they jumped in high heels.
Revealing mistake: When Bluto first watches the sorority girls on his ladder, he moves to the left screen. As he is moving the ladder, the curtain in front moves too. The glass pane is not present, likely to prevent reflection of the camera.
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