- Billing Displacement: Rachel McAdams gets top billing behind Gyllenhaal and Forest Whitaker despite being killed off in the first act.
- Completely Different Title: Due the term "Southpaw" not being widely used, some countries went for a different title. In Brazil, for example, the movie is called "Nocaute" (Knockout).
- Divorced Installment: Was originally written with Eminem in mind for the lead and was a sequel to his film 8 Mile. However, he turned it down and the film was re-written, with Jake Gyllenhaal taking place as lead though the boxing element from the original script stayed. Never the less, Eminem still stayed on aboard for the soundtrack.
- Doing It for the Art: Antoine Fuqua had no money to pay James Horner to compose the film, due to the film's short budget. However, Horner did not care as he loved the idea of the film, and he decided to do it for free.
- Dyeing for Your Art: Since Jake Gyllenhaal had to lose thirty pounds for his role in Nightcrawler, he only needed to gain fifteen pounds for the role of Billy Hope by training twice a day, seven days a week.
- In Memoriam: Some prints of the film, at the end of the credits, have "In Memory of Our Friend James Horner," who died in a plane crash days before the film opened for release.
- Missing Trailer Scene: In the trailer, there was one scene that shows Leila getting in an argument with Billy before slamming a milkshake she was holding onto the ground and storming off. This scene was never shown in the movie.
- Star-Making Role: For Oona Laurence even though shed already been acting for a few years at that point.
- What Could Have Been:
Trivia / Southpaw