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.
Revealing mistake: In the scene where Mr.Wilson slips on the bath water and falls, after he does the split you can see his right leg is fake.
Factual error: When the shop vac shoots the golf ball at Mr. Wilson's gonads, it only affects him for around ten minutes. The shop vac was able to shoot a glob of paint over 50 feet in the air so the speed and impact of the golf ball would have done much more than put Mr. Wilson in extreme pain for a short while. The impact would have likely caused internal bleeding, swelling, causing him to vomit, probably a rupture, requiring him to be hospitalized. There is no way he is walking away from that, he wouldn't even be walking at all.
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: One of the first shots of Switchblade Sam is a closeup of his face, in which the viewer can see he is wearing contact lenses. Kinda hard to believe this homeless-type villian with blackened and missing teeth, old torn clothes and unwashed stringy hair maintains contact lenses. (00:18:20)
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.
Plot hole: Towards the end of the movie we see Mr. Wilson peek out from his front porch, surprised to see Dennis biking back home with his wagon carrying Switchblade Sam, tied-up and partly unconscious. Either Dennis must have some hidden super-strength or knew how to tweak the laws of physics there is no way that an eight-year-old kid of his build could carry an adult of that size to his wagon let alone easily bike home with Switchblade Sam's weight. Plus being an experienced criminal like Switchblade Sam he may have offered some resistance in being caught doing so much less submit to a mere kid. (01:50:00)