Dawson Casting: Marilyn Ghigliotti was 33 when she played Veronica, a student in her early 20s.
Enforced Method Acting: An unintentional case. According to the commentary, Jeff Anderson apparently screwed up Randal's "The Reason You Suck" Speech that was directed toward Dante multiple times, so that in the finalized scene when Randal snaps Anderson's rage is 100% real, fueled by how many times he had to do it over.
Executive Meddling: One of the good examples. The original Downer Ending had Dante being murdered by a holdup man but it was never included in any versions of the film. The change was motivated by film guru John Pierson and Brian O'Halloran (who plays Dante), not any execs, though.
Hey, It's That Voice!: The show features the voices of Alec Baldwin, Charles Barkley, Sen. Al Franken, James Woods, and Gilbert Gottfried playing the parts of both Jerry Seinfeld and Patrick Swayze (though, for legal reasons, the character could neither look nor sound like Swayze himself).
Frequent Disney VO April Winchell shows up a few times, as either one-offs or the recurring TV news reporter, Tovah Hernandez-Carlson.
The scene during the wake, which was later adapted as a comic and an animated short but was never going to be in the movie at all. A better example would be a scene in Big Choice Video, where Randal interacts with a hypercompetent employee named Chet. The footage was burned on the editing machine and mangled beyond use. What little footage survived was used before Caitlin first appears. Some of the audio survived, and can be heard in Snowball Effect, the Clerks making-of documentary on the tenth anniversary DVD set.
But, on the plus side, all six of the episodes have been released on DVD and Cartoon Network does air all six episodes on occasion. They even aired the second episode with the scene where Randall and Dante watch a Schindler's List parody called Flintstone's List. In the ABC version, they only showed Randall and Dante watching the TV with the sounds from the movie playing. On Cartoon Network and the DVD, there's an actual clip from the movie.
Other networks (most notably Adult Swim and Comedy Central) were interested in picking up the show for new episodes after ABC canceled it, but ABC refused to sell the rights and has simply sat on them. We're lucky we at least got reruns on those networks and a DVD.
Combine this with the fact that when they were pitching the show another network (often thought to be UPN) wanted to basically let them do what they wanted and make them their flagship show (similar to Fox and The Simpsons). Smith compares it to someone outright asking you to be the big fish in a little pond, but since ABC was a bigger network they signed with them. Smith and Mosier admit in the commentary that this wasn't their finest hour.
The reason ABC greenlit the show then abruptly canceled it was due to the fact that there was a rush by many networks in the late '90s/early 2000s to have adult-themed animated primetime shows following the success of South Park. However, between the time episodes began production and when they aired, Who Wants to Be a Millionaire? became a breakout hit and ABC sought to bury Clerks so it could give an extra half-hour to the game show. The moral: it doesn't matter how much time and effort you put into a TV show, networks are fickle with desires that change on a whim. Though Kevin Smith has admitted that he loved hearing Regis say, "Up next: Clerks." All of two times.
Serendipity Writes the Plot: The Quick-Stop wouldn't let Smith shoot inside during the day, so he had to film all the interior shots at night. To hide this, he came up with the idea of someone putting gum in the locks on the shutters, necessitating them staying closed all day.
Randal exploits Jay by making "snoogans" a Catch Phrase, leading to Jay becoming a recording artist.
Randal buys KITT at a used car lot. The car becomes jealous of Randal's relationship with Dante and attempts to kill Dante and impersonate him at the store (a reference to Single White Female). He manages to trick everyone except Dante (and most likely Silent Bob) with his thin disguise.
Jay joins a boy band. Silent Bob is sad that Jay is gone, so Randal and Dante bring Bob in to work at the Quick Stop. He turns the store around and it becomes a great success. Silent Bob is given a piece of gum and says, "Mmm, Juicy Fruit," which shocks Dante and Randal (a reference to the Chief in One Flew Over The Cuckoo's Nest).
Allegedly in the 7th or 8th episode, Dante and Randal would have added to their cast of friends a boy ward, named Robin, in a direct reference/parody of Batman's own "Boy Wonder", though like Lando, would probably not have been featured prominently. This plot element was mostly likely not intended to actually be part of the series as it was described by Kevin Smith after mentioning that Paul Dini, best known for his work on Batman: The Animated Series, had also worked on the Clerks cartoon.
The DVD commentary briefly mentioned an episode idea where Ben Affleck would play the King of Canada. The episode premise was also described as "the Aladdin parody."
Notice how much Leonardo Leonardo looks like Hans Gruber? That's because they originally wanted Alan Rickman to voice him.
The show was very nearly picked up by HBO. If it had, all swearing, sex, and drug-related references would've been allowed to air without censorship.
Originally, the series was considered for UPN, with an initial run of thirteen episodes and the promise of heavy promotion. Production instead went with ABC (rather than being "the big fish in a little pool"), which they conceded was a mistake.
Over the years, there has been talk of a DTV movie Sell Out, which basically would've had the cartoon characters making a movie based on the actual movie. Assorted behind-the-scene developments (such as the Weinstein Brothers leaving the studio) haven't helped move it forward.
Write What You Know: The scene in episode 4 where Dante and Randal are bad-mouthed by the basketball players in the elevator ("Were they talking about us?") is based on a real life event that occurred to the writers while working on this show.