Production Oddities
The Tarantino Feud: Quentin Tarantino wrote the original script but infamously disowned the film after Oliver Stone heavily rewrote it. Tarantino was so upset he reportedly tried to block the movie from being made and told actors he would never cast them if they took roles in it.
Real Prison, Real Inmates: The prison riot sequence was filmed at Stateville Correctional Center in Illinois, where actual inmates served as extras. For the first two weeks, these inmates were even given rubber weapons for the scenes.
A "Coke" Disaster: Coca-Cola approved the use of their famous polar bear ad, mistakenly believing it would be shown during a scene where a character watches the Super Bowl. Instead, Stone juxtaposed the "cuddly" bears with extreme violence, leaving the company's board of directors furious.
Psychological Warfare on Set: To maintain a frantic energy, Oliver Stone would blast African tribal music and loud industrial rock at top volume between takes. 
Cast and Character Facts
Harrelson’s Connection: Stone cast Woody Harrelson because he felt the actor had "murder in his eyes". In an eerie real-life parallel, Harrelson’s own father was a convicted hitman.
Improvised Filth: Comedy legend Rodney Dangerfield played Mallory’s abusive father. Stone didn't give him a script; he simply told him to be the "father from hell" and allowed Dangerfield to write all of his own "filthy" dialogue.
Broken Bones: During a scene where Juliette Lewis slams Tom Sizemore's face into a wall, she actually broke his nose. Separately, the Director of Photography broke his finger while shooting a POV scene. 
Technical "Overkill"
Editing Record: The movie contains nearly 3,000 cuts, which is roughly five times the amount of a standard film from that era. It took nearly a year to edit because it utilized 18 different film formats, including 35mm, 16mm, Super 8, and video.
Green Blood for Censors: To secure an R rating instead of an NC-17, the production reportedly used green blood for some vampire-like effects to make the gore seem more "satirical" and less literal to censors.