Revealing mistake: Don't worry, Aunt Sally isn't really dead. She's still breathing in the shots where people come to look at her in her coffin.
Revealing mistake: Although set in the 1960's, the school bus lettering was Arial, a font not invented until 1983.
Revealing mistake: In the beginning of the movie, there is a shot of Largeman opening a medicine cabinet. If you look on the mirror as he opens it, you can see the cut out of where the camera is hidden in the wall. This is pointed out by Braff in the commentary.
Revealing mistake: When Mary of Guise is found dead by her nephew, he lies her down and puts his head on her chest. As he comes down, her eyes slap shut! If she were dead, they would remain open.
Revealing mistake: During the scene in which Lou forces himself on Kim in the lunchroom, donning a black ski mask, Kim's brother Alex jumps up to his sister's defense. Just before Alex throws his first punch, Kim screams "Michael," referring to the actor's real name Michael Tough. Kim then correctly screams "Alex," the name of the character he was playing in the film.
Revealing mistake: Tire tracks from the camera vehicle can be seen in the grass in the brief shot of polo players moving toward the camera.
Revealing mistake: When in Boston, while walking through the park, Susan and her mother are obviously walking on a treadmill because the park background is moving way too fast for their walking speed.
Revealing mistake: Right after "Without Love", we see Edna answering the phone. If you've seen the movie as many times as I have, you'll see that this is a recycled shot from earlier when Mr. Pinky calls.
Revealing mistake: When Chris and Jamie are on their day date in the diner, Jamie has a grilled cheese sandwich with ranch on the side. However it is obvious that the bowl of ranch is empty. (00:33:10)
Revealing mistake: Dooley Wilson (a singer, drummer, and leader of his own touring band) who played "Sam" in the film, couldn't play the piano. A pianist was playing offstage while he sang the songs. If you look at Dooley's hands whenever he is 'playing' the piano, you can see that he is not playing the proper notes. (00:25:30)
Revealing mistake: Hamlet is killed and in the next scene is being carried with his arms stiff to his sides. The men carrying him go over a bump, jolting Hamlet, and he blinks.
Revealing mistake: At the beginning of the movie a hamburger is thrown in hopes that a dog will "fetch" it. When the dog does not do so the hamburger is retrieved twice, in perfect shape, to be thrown two more times. On each throw its various parts would have landed individually all over the place and not been throwable again.
Revealing mistake: Jorge misses scanning the barcode on the tire by several inches at the end of the competition, but it rings up anyway.
Revealing mistake: When David is sitting in L.E. at the end and the camera is zooming in on his face his mask changes. In the very last shot of him just about to pull it off it's much wider/looser so it'll come off easier.
Revealing mistake: During the wedding scene, while the narrator is talking, you can see the string that will pull the left table apart. As said in the commentary, the filmmakers weren't going for the idea of people pulling the tables apart, but rather some other force parting them. (01:27:40)
Revealing mistake: Apparently filmed inside an old warehouse, heavy masonry walls can be observed during some scenes inside what is supposed to be a spaceship.
Revealing mistake: When Gary and his friend are drinking Bud in the bar, the Bud label moves between shots to keep facing the camera. Gary doesn't touch the bottle.
Revealing mistake: When Phil drives the truck off the cliff, you can see right through the grill of the truck (meaning there is no radiator or engine), and the transmission is also missing, as you can see when the truck is falling upside down. Without an engine or transmission it would be hard to drive off a cliff.
Revealing mistake: When Francis emerges from the cave, he stands in the foreground, with the distant mountains and sun in the background. The sun creates a lens flare, which is an internal reflection within the camera lens. Yet Francis is in front of the flare. This shows that the actor was composited with the background via a traveling matte process.
Revealing mistake: When Herbie is being pulled from the truck at Crazy Dave's, Herbie goes into reverse and the camera does a closeup shot of one of the rear wheels reversing. If you look carefully, you can see an extra set of small wheels behind the tyre which are obviously propping the car up enough so that the cars proper wheels will only skid and not get any traction.