Development Gag: In Best Prank Ever, Jimmy briefly wears a bowtie, much like he did in the pilot mentioned below and this concept art.
Around the Heinous mansion, portraits of Lucius in his original suit from the pilot (which is brown-colored instead of black) can also be seen.
The Mysterious Mr. Ten has even more references to the concept art, with pictures of the pilot version of Heloise (who is essentially the same but with grayer, more dead-like skin) on Mr. Ten's wall.
Executive Meddling: The show was changed slightly from the original pitch before the final product was greenlit. However, a pilot was made containing many of the infamous original concepts, notably including Miseryville being explicitly stated as Hell, rather than just an implication. See What Could Have Been below.
Fandom Nod - Edward Kay has admitted that Panda-Monium was a Shout-Out to Heloise's fans.
"I'm successful!, I'm popular! That's right, I said popular."
When Jimmy is fighting the clowns in Power Squid and Spaghetti Beezy, listen closely and you can hear Jimmy shout "Hadoken!" at one point.
A classic Rocky styled montage is featured in Miseryville Marathon.
In The Outsiders, Lucius has a fantasy where he is dressed up as Galactus.
In Better Sweater, after Heloise tries catapulting up to Jimmy and hits the wall, she mutters a dazed "Missed it by that much," likely a refrence to Get Smart.
In The Hooded Chicken, Heloise briefly strikes a pose made famous by Marilyn Monroe.
What Could Have Been: Jimmy was originally going to die in the first episode and end up in Miseryville thanks to an administrative error. His Dead to Begin With status would have allowed him to do some morbid things, like pull bones out of his body (though he actually DOES pull the bone out of one of his arms in the episode Monster Mutt) and unscrew the top of his head.
Heloise would have been a Serial Killer that was hit by the same bus as him and would have been arranged to torment Jimmy, only to fall in love with him.
Samy is a Starscream who would overthrow Lucius if he had the chance.
Also on Samy: "I don’t think of him as either gay or straight, just someone with a pathetically desperate need to feel needed and more important that he is. He’s a typical fascist – cowardly and obsequious to those above him, and a tyrant to those “below” him."