Revealing mistake: When Cindy pulls Mrs. Norris and places her hair in front of the fan, hair can be seen flying in the air before Norris' hair gets in the fan. (00:16:35)
Continuity mistake: When Tom grabs the gun from his box we never see him closing the box. Yet in the next shot as he shoots himself the box is closed. (00:35:05)
Continuity mistake: When Tom and the kids are in the minivan and Marvin is trying to get in, the door lock changes from shot to shot, even when none of them touch the door handle or unlock button. (00:36:55)
Continuity mistake: When it cuts to class when the kid is telling the story about the duck, we see the President sitting beside a table. On the table we can see his glasses and a plate. When it cuts a book has suddenly appeared. (00:38:15)
Continuity mistake: In class when the agent tells the President the aliens are attacking and the President responds "You mean, right now all of these children's parents could be dead?", there are some items on the table next to the President. When it cuts the items has changed position. (00:39:00)
Continuity mistake: During the classroom scene, the position of the President's head changes between shots. For example, he is looking at the class, then his assistant, then the class again between shots. (00:39:00)
Factual error: Cindy and Brenda watches the news camera footage of Detroit before and after the tripod attack. But the city shown is actually San Diego. (00:40:45)
Continuity mistake: When Cindy and Brenda are in the car approaching "The Village", Cindy pours coffee on her head to stay awake. In the next shot of her, the coffee is gone, her head is dry. (00:45:00)
Continuity mistake: When the President is at the UN and he puts the glass of water down on the red button after he takes a drink the glass is nearly empty, but when he places it on the button it is half full. (00:53:55)
Continuity mistake: In the village when one of the "monsters" sneeze and the other one gives him a tissue, it cuts to Cindy and Brenda gasping. The "monsters" then turn around and the one with the tissue has its mask hanging outside of his cape. When it cuts the cape is covering the whole mask again. (00:58:55)
Continuity mistake: During the Village parody scene with the man with the knife in his heart, Cindy leans over and talks to him and winds up pushing the knife completely into his chest by leaning on him so that no metal part is visible. In the next shot, the knife is back as in its original position. (00:59:55)
Revealing mistake: When the little boy is killed at the boxing match in the flashback, you can tell the Don King-looking-guy is falling onto a dummy, and not the real boy. (01:00:50)
Continuity mistake: When the killer (Jigsaw I suppose) is releasing Tom's children from being hung by their hands, right when they get down the rope holding them up disappears.
Continuity mistake: When Jigsaw comes in on his tricycle for the first time, his handle bars are straight, but when Zoltar enters, his handlebars are curved.
Visible crew/equipment: When Tom Ryan is racing home to meet Robbie and Rachel, as he gets into his driveway he opens his door and you can see the camera reflected in the rear view mirror for a split second.
Revealing mistake: Halfway through the movie, with the crashed plane, the aircraft engine is actually running backwards.
Answer: Ads are often made months before the final editing of any movie. Therefore, it's not unsual for something to be in an ad but not in the final movie (or to happen differently, or a different shot, or different music, etc.).
Myridon