Continuity mistake: When the butler is serving up the turkey, he holds up a plate full of what looks like turkey, salad and mash. When he puts it on the table, the plate just has turkey on it.
Revealing mistake: When the guy with the weird hand is serving dinner, you can clealy see through the cuff on his left arm that his real hand is there.
Visible crew/equipment: At the end of the film when the bird is pooing on the wall, you can clearly see the pipe leading up to the fake bird which is making it do this.
Continuity mistake: When the two guys are facing off in their wheelchairs the doves fly by and poop on the bald guy's head. But as he starts to move the poop is gone. But in the next shot it's back again.
Continuity mistake: After the man in the wheelchair first breaks his glasses. It is then in perfect shape in one scene, then broken again in another scene. This continues on for a few minutes.
Continuity mistake: In the scene of the dinner, Hanson cuts the turkey, but in a scene when Dwight is speaking, the turkey is perfectly OK.
Revealing mistake: When the couple split the pole, they get hit by a bus. As the shot cuts to a wide shot, and if you pause in the right place, you can see the couple have been replaced by dummies, just before the bus hits them. [This isn't a "mistake" -- this is just dissecting a movie, frame by frame, until you can spot the special effects.]
Continuity mistake: When Dwight is handing everybody goggles, cup walkie talkies etc., the red numbers on the machine nearby keep changing in most shots. You can clearly see "02.38.58" and it is counting down. Then in the next shot it is "02.39.48" and is counting up. Then it is "02.17.00" etc.
Continuity mistake: In the opening scene when the priest is playing the piano, in close shots of him you can see the back of the musical sheets sitting on the piano, and they are just pages of musical notes. Yet between two shots a book which says "Eckstein" appears. Then in the next close shot the "Eckstein" booklet isn't there and just musical sheets are visible.
Continuity mistake: When Cindy and Buddy are in the cool room and she is talking on the cups, you can see what appears to be blood on the bottom of her pants. In a scene cut from the film (it's on the DVD), Cindy covers the floor in blood before they get pushed into the cool room (spoofing Hollow Man). Since scene wasn't in the film, she shouldn't have the blood on the bottom of her pants.