Other mistake: During the credits of the movie, after Morris Day points to Jay and Silent Bob, there is an Asian guy jumping up and down at the far right hand corner. The shot switches back to Morris Day and when the shot goes back to Jay and Bob the Asian guy is wearing a light beige hat. The extra submitted this entry himself - he was wearing the hat during the shots. Then after several takes someone in costumes noticed it was a major logo ("No Fear") so he had to take it off. The shot was used before he had his hat on. (01:36:00)
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).