Revealing mistake: When Stu is tempting the sniper to see if he will really shoot, the sniper shoots the phone booth, shattering the window, and supposedly hitting or grazing Stu's right ear. When Stu lifts his head, you can see his bloody ear. A few shots later, Stu's head is turned so you can clearly see his fresh wound. If you look closely, you can see Stu's real ear folded back beneath the fake latex one. This happens several more times in the next few minutes of the movie. (00:31:40)
Revealing mistake: Near the start, in the first few minutes, in several shots you can see barriers that are separating the crowd from the set. They are right back, so they couldn't be too visible.