Continuity mistake: At the university in the beginning, the blackboard on the wall changes position in every shot - once it's pushed up, next shot it's at the bottom. Several times. (00:04:10)
Continuity mistake: In the construction site scene, you will notice the length of Ben's cigarette fluctuates several times. It actually gets longer/shorter as Ben smokes it.
Continuity mistake: When they are crossing the bridge to go back into Boston, after leaving the bar in Cambridge, a Red Line subway train is shown and then in the next shot there are several buildings in the background as they cross the bridge. These buildings are in Boston, but this shot has them driving away from the buildings, which means they would be re-entering Cambridge. (00:22:15)
Continuity mistake: When Ben and Matt are sitting at the construction site drinking a beer, Ben takes a sip of his beer with his right hand in a close up. The next shot shows him holding the beer in his left hand, flicking a cigarette with his right. (01:43:55)
Continuity mistake: In the scene "When Did You Know?", when Will tells Sean that he read his book last night, an aerial shot looking down at them shows Will with his foot up on the table. But two shots later Will has his foot up on the chair instead. The chair in the first shot would also be too far away for Will to reach from his own chair, so that would have been moved in the later shot for him to reach. (01:00:30)
Continuity mistake: In the scene where they are all gathered in the bar together Ben is telling the story about his uncle. Throughout the entire scene, Ben has nothing in his hands but as he comes to the end of the story there is a camera shot over his right shoulder showing him holding a cigarette. However, when the next shot moves to in front of him, the cigarette has gone again. (01:07:45)
Continuity mistake: At the end of the film during Will and Sean's last meeting, there is a shot showing both Will and Sean and we see Will with his hands in his lap. It then cuts to a shot over Sean's shoulder and we see Will, but now his hands are raised and his elbows are on the arms of the chair. (01:53:25)
Continuity mistake: The problem on the board that Will solves changes slightly. The scene is a bit apart but you can see the huge change. Once it reaches all the way to the bottom. Other time, it only reaches halfway. One time it had (I think) 1,2,3,and 4 and other time only 1,2,3. (00:05:00 - 00:08:25)
Continuity mistake: The first few minutes of the movie contain classroom/lecture hall scenes - the walls are a very distinct dark brick, and the seats an uncanny orange. This decor is at the University of Toronto, not at M.I.T. where the movie is set. (00:03:30 - 00:04:30)
Continuity mistake: Near the end of the film after Chuckie realizes that Will is gone, he turns and heads back to the car. As he first goes down the steps and down the walkway you can see Morgan already sitting in the front passenger seat. As Chuckie approaches the car, however, you then see Morgan excitedly jump out of the back seat and run around the car to jump in the front seat.