Other mistake: After Jay and Silent Bob catch a ride in the van with the quartet of diamond thieves and Brent, they say they are the Kansas State chapter of SAAC and they are headed to the Provasik Animal Testing Lab (located next to the Colorado Diamond Exchange, so therefore most likely in Colorado). The Mooby's restaurant is located in Kansas City, Missouri (as stated on the sign visible when Jay wakes up from his Scooby Doo nightmare). The Kansas State University campus is located in Manhattan, Kansas, which is about two and a half hours west of Kansas City. If the diamond thieves are headed west, to Colorado, then why are they stopping to eat nearly three hours out of their way to complete their mission? Even though the thieves aren't really from KSU, Brent was their patsy, meaning that he wasn't in on their plot, and therefore must himself have been from KSU; he would have no reason to lie to Jay and Silent Bob, given he tells them the truth about their mission (as far as he knows).
Continuity mistake: When Justice is talking to the other girls (mainly Sissy) up at the balcony of the motel room, there are some clouds in the sky. However, between shots suddenly the sky is completely clear. (00:48:41)
Continuity mistake: In the comic store, "Brodie's Secret Stash," if you press "pause" right when Brodie reaches for the Bluntman and Chronic comic book to show Jay and Silent Bob, you will see that he is right across a comic entitled "Cable." To the right of that is another comic, "Captain America," and if you can recognize it, to the right of the "Captain America" comic is a "Captain Marvel" comic. If you look behind Jay, you can see the same exact thing.
Continuity mistake: Near the end, when Jay and Bob are revenging against all those who dissed them on the internet, they read from a stack of computer printouts of the insults. In one shot, you can clearly tell that when Jay repeats a paragraph of an insult to a kid, he reads from what looks like a few words on the paper. In fact, the entire paper looks like a list of the same label repeated.
Continuity mistake: In the scene where Jay and Silent Bob go to speak to Holden McNeil (Ben Affleck), Ben shows them the internet site, moviepoopshoot.com. Well in the address bar it is clearly not http://, but file://, which means the site was being shown locally on the hard drive, not with a connection.
Continuity mistake: During the gunfight at the end Will Ferrel's and the vixen's model of firearms change a few times.
Factual error: Near the end of the film, during the big shootout scene, there are 2 girls vs. Will Farrell. If you play the movie in slow-motion, you can see that the guns the girls are shooting continue to shoot fire out of the barrel several times, even though the slide is locked back. When a handgun's slide locks back, the magazine is empty. How could they still be shooting when there are clearly no bullets left?
Continuity mistake: In the beginning when they are in the comic book store, they are asked if they were paid for likeness rights for the comic book and Jay and Silent Bob said no. Towards the end of the movie when silent Bob gives Banky the long legal spiel during the shootout, silent Bob tells Banky they had an agreement on the likeness rights and they were supposed to be notified before it was transferred to another medium. So, in the beginning of the film they said they had no agreement on likeness rights and towards the end they said they did.