Revealing mistake: When members of Fight Club are starting fights, watch VERY closely when the guy with the hose sprays the vicar - you can see the camera wobble slightly (looking on the left of the screen at the top of the fence is a good reference point). This happened because the cameraman started to laugh. (01:15:10)
Revealing mistake: When Brad Pitt is cutting out newspaper articles about Project Mayhem's mayhem, the headlines refer to their deeds, but the actual writing in the articles has absolutely nothing to do with the movie. They seem to all come from one article about some town council meeting. (01:21:10)
Revealing mistake: Just after the car crash scene when they are all crawling out of the car, a large drop of rain is seen in the very top left of the picture. The drop is too large and falls too slowly to be an actual drop of rain. This shows there is an umbrella type thing protecting the camera from the water. (01:41:10)
Revealing mistake: In the scene where Ed Norton discovers that Meat Loaf's character has been killed, you can clearly see him breathing in the long shot while he is lying dead on the table. (01:46:15)
Revealing mistake: Towards the end of the film, when the narrator and Tyler are fighting in the basement, Tyler slams the narrator's head into the wall by the van. For a split second, before the shot changes, the 'concrete' wall visibly buckles inwards. (02:03:35)
Revealing mistake: Before the final fight scene between Tyler and the narrator, Edward Norton points a gun at Tyler telling him to step away from the van. Tyler breaks the key and throws it away. When this shot is seen through the security footage, then in the far left security computer screen we can see that there is already a gun shot hole in the back window of the van, and there is also some random guy walking in the top left corner of the screen. (02:07:35)
Revealing mistake: When Brad Pitt throws Edward Norton down the stairs, there's a shot close to the bottom of the stairs in which you can see the stunt-man's face. (02:09:15)
Revealing mistake: Towards the beginning when Edward Norton is ordering out of the catalog, he is walking into his kitchen. Look at his shadow to his left, it doesn't change distances indicating a blue or green screen.
Revealing mistake: When the "waiters" leave the restaurant after assaulting the old police man you can see that Meat Loaf's pants fall down revealing the wrinkled fat suit he is wearing underneath.
Answer: Most likely they either went back afterwards or chased the security guard off somehow. It's also possible with all their inside connections that they stole the body from police custody.
Greg Dwyer