Demand Overload: The Simpsonize Me website that was launched concurrently with the movie saw so much traffic during its early life on the internet that it frequently saw downtime. The fact that the site also utilized high-quality animated graphics didn't help either.
Throw It In: Hans Zimmer produced a choral version of Spider-Pig for the soundtrack just for shits and giggles, but the music worked so well with the epiphany scene that the producers placed it there. Turns out to be an overblown gag that snowballed out of control, as FOX also had to secure the rights to the song, and Zimmer had to redub it into different languages! It even became a hit single in several countries, making it, at a scant 61 seconds, the shortest recording ever to chart.
Vanity Plate: Gracie Films' appears at the end of the film in a homage to the television series; this is the first and only time to date the plate has appeared in a feature film produced the company. (A version of the logo would later appear in the theatrical short Maggie Simpson in: The Longest Daycare.)
What Could Have Been: A Simpsons movie has been planned since the 1990s, but the writers couldn't figure out how to stretch the plot to feature-film length. Episodes that were considered for a feature-length movie are "Kamp Krusty" (which became the first episode of season four) and "Bonfire of the Manatees" (the first episode of season 17). Before his death, Phil Hartman had expressed interest in doing a live-action Troy McClure film.
Kelsey Grammer's Sideshow Bob and a family therapist voiced by Minnie Driver were originally going to be in the film, but for some reason their scenes were cut.
Hank Scorpio from "You Only Move Twice" was going to be the villain instead of Russ Cargill. Hell, they not only share the same voice actor, but the same voice. They decided against it because Scorpio was found to be too likeable to turn into an all-out villain.
The film was going to have more PG-13 language than just Marge's one "Goddamn."
Erin Brockovich was to make an appearance in the movie.
Apparently, it was originally supposed to be President Schwarzenegger who managed to KO Cargill (with a tree branch!), instead of Maggie dropping a boulder on him.
Instead of going to Seattle, Marge and the kids were going to go on The View where Russ Cargill shows up, gets in a shotgun duel with Joy Behar, and has the Simpsons arrested.
Isla Fisher and Edward Norton were going to make appearances but their scenes were cut.
And, furthermore, though many of these scenes were animated, and the producers claim the film changed so often that at least another film's worth of material made it to storyboard and/or animatic, only a few minutes of dull deleted scenes appear on the DVD.
Originally, Marge, not Grandpa was supposed to be the one to have the prophecy at church, but the writers admitted they were a little disturbed by the fact the entire family would just ignore when someone like Marge would completely wig out, so they changed to the character that everyone ignored anyway.
Maile Flanagan was the original voice actress for Colin, but she was let go at some point in production and Tress MacNeille re-recorded the character's dialogue.
A lesser example: Marge's line about what Homer was doing with the pig's "leavings" began as what Matt Groeaning described as "the world's longest shit joke," with Marge giving a Long List of different euphemisms for poop, none of which would register to Homer.