Big-Lipped Alligator Moment: During editing, Bousman realized there was no place to actually fit in the song "In All My Dreams I Drown" that would make sense... so they put its scene in the credits, since the song was too good to cut.
"In All My Dreams I Drown" has proven to be popular.
Lucifer's Villain Song "Grace for Sale" has a hypnotic beat that sticks in your head for days. Doubles as Crowning Music of Awesome, considering he's declaring war on God himself for neglecting his creations. "Fa la la! It's off to hell we go..."
"You're a tough little tadpole to love..."
"Good Little Dictation Machines" from The Devil's Carnival Part 2 had the honor of being an ear worm before the movie had even been filmed, thanks to Terrance Zdunich and Saar Hendleman's performance during the Repo/TDC tour.
Terrance Zdunich's portrayal of Lucifer in the song "In All My Dreams I Drown".
The Scorpion also seems to serve this purpose. And how.
Friendly Fandoms: With Repo! The Genetic Opera. Both being Cult Classic musicals with similar themes and aesthetics, and a lot of the same people involved, it makes sense that these fans would overlap. More than overlap, actually — if you're a fan of one, it's practically a guarantee you're a fan of the other.
Ship Tease: Various times, between various couples, particulary Wick and the Magician, who spend an awful lot of time hanging off each other (which is Hilarious in Hindsight considering that their characters in Repo! (Luigi and Shilo) were quite the popular ship themselves)...
Bill Moseley and Alexa Vega are pretty chummy◊ backstage as well.
They Wasted a Perfectly Good Character: Two of the Woe-Maidens are Captain Maggots and Contessa, both of whom have actually spent time as part of a circus. When you have a woman who slings flaming hula-hoops around herself and another who specialises in aerial silks, and your movie is circus-themed, you really should find a way to put that in! There's actually a brief shot◊ of Maggots and her hula hoop in the beginning, but that's about it.
Tamara? You're supposed to ask him how you know he won't stab you before he's got you tied to the wheel!
Miss Merrywood. Woman practically personifies Too Dumb to Live! She replies to a sign saying "Take only what you need" with a smirking "I need it all! Ahahaha!" and steals all the jewels... and then ogles her appearance in a mirror (conveniently missing the "Do Not Trust This Person" sign that points at the mirror)!