Continuity mistake: When Dennis is in Mr. WIlson's bathroom after his bath, pouring mouthwash in his nose spray, Dennis has toothpaste all over his chin and mouth. When they show Dennis from the side, it is gone. From the front it is there again. This happens several times in the shot.
Continuity mistake: In the scene where Dennis and his mother are on their way to Margaret's house, the lock on the passenger's side door is locked and then unlocked when the camera view changes. (00:11:40)
Continuity mistake: During the bathroom scene, Dennis pulls the cap off the nasal spray bottle and throws it. When the scene cuts to him holding the mouthwash, the bottle is sitting with its cap on, and immediately after the cap is off again.
Continuity mistake: In the bedroom scene, Dennis' overall buckle is normal. But after he pops the cap off the aspirin bottle, it's pushed all the way to the side. The next shot shows it as normal again.