Deliberate mistake: The Stan Lee cameo contains a factual error; he's checking out of the Mallrats script and rehearses his lines, but despite coming out later in the year, the movie (and therefore Lee's cameo) had already been filmed, since movie shooting ended in April. Unless we have to assume Stan Lee carried with him all the time scripts of movies he'd been in and randomly read his scenes aloud. (00:30:45)
Deliberate mistake: At the speed trap, Tom points his radar gun at a turtle. From his perspective, he seems to be looking through some sort of scope with cross hairs and numbers on it at the turtle to line up the beam. However, when he then looks at the radar gun at the speed indicated as 1 MP/H, there is nothing on the gun where he could have been looking through such a scope. At best, the top of the run appears to have plastic iron sites. The scoped view would not have been from the device itself perspective since it would not need cross hairs or numbers on said cross hairs as that would be for a person looking through it to have reference points. This scope view of the turtle was most likely just added by the film makers to have a better visual representation of Tom aiming the radar gun at the turtle. (00:04:29)
Deliberate mistake: The "CPR doll" smashed through the car's windshield, knocking out two-thirds or more of the glass. The doll would not weigh enough to break through the windshield (and maybe not even cause a hairline crack). The excessive amount of glass smashed out and the distance the doll flew before landing on the highway were obviously exaggerated for dramatic and comedic purposes. (00:47:00)
Deliberate mistake: The prisoners in the dark dungeon would not be able to read the letter (to Miss Dudley from Jason Flemyng that the carrier pigeon had) or see such a clear reflection by pouring water on the stone floor and holding the letter (written backwards) by the water. The room was suddenly lighter when doing so, but it wasn't clear where the light was coming from. Also, sunlight coming through the small window would not beam straight down along the wall. (00:07:23)