In the scene where Dave finally comes clean, when Angela is shown the photo of Ethan's shrine to her, her image on the screen in the photo is moving.
In the scene where the guys are getting Carl to fill out the Physics test, they pass him a joint, but it the next shot Sam is holding it and in the shot after Carl is passing it back to Sam.
When Jack is copying files off Angela's computer, he inserts an Iomega Zip disk, but the screen says "Drive: A". Drives A and B, when present, are always floppy drives (either 3.5" or 5.25"), never Zip drives.
When Ethan is video taping Angela and Dave next to the pool, he says his line next to the camera. When he is showing the video tape to Sam and Jeff, you can hear Angela's and Dave's voices clear from far away but you never hear Ethan's line.
When Ethan is confronted by Angela with the hair doll that he made, she holds it up and says, "Is this a hair doll?" her lips are clearly not moving.
In the scene where Ethan is being forcibly escorted from the law firm lobby, when he turns his back, his shirt lifts up, exposing a wireless microphone transmitter clipped to the back of his pants.
In the last shot of the film, as credits begin to roll and Ethan has finished singing his song, you can see several feet of the dolly track on the floor of the diner. One of the extras sitting in a booth even comes close to tripping over it. In the wide screen version of this film the dolly track is not visible.