The film is titled "Very Bad Trip" in France, per a bizarre trend that had French marketing departments re-title films — still in English — based on previous successful and unrelated comedies, Very Bad Things (in France at the time) in this case. Then plenty of unrelated comedies that followed would be retitled with "Trip" in France, such as Get Him to the Greek ("American Trip").
In Russia the series is known as "The Bachelor Party", despite the third movie having no bachelor parties.
In Latin America, the trilogy is known as "¿Qué Pasó Ayer?" (translated as "What Happened Yesterday?").
In Brazil, it's "Se Beber, Não Case" (If You Drink, Don't Marry).
Polish version went for a pun-based title with "Kac Vegas" ("kac" being the Polish word for hangover). Oddly, they kept the title even as sequels moved to different locales - second film became "Kac Vegas in Bangkok", and third was simply "Kac Vegas III".
Corpsing: When Stu wakes up and finds the tattoo on his face, Bradley Cooper breaks into a giggle at one point.
Star-Making Role: Basically all three leads. Zach Galifianakis got the most immediate attention; Bradley Cooper got equal billing with Liam Neeson in the film version of The A-Team; Ed Helms has a number of lead comic roles in production and received an increased role on The Office (US). Additionally counted as a Star-Making Role for Ken Jeong, who saw himself cast as a supporting player in a number of major Hollywood films immediately afterward. He also ended up becoming a main cast member on Community.
Ed Helms and Zach Galifianakis's songs ("Stu's Song" and "Three Best Friends", respectively) were improvised.
Galifianakis improvised the line "I didn't know they gave out rings at the Holocaust". The initial line had Alan making a sexually inappropriate comment about Stu's mom, but Zach thought up the new line for two reasons: he wanted to show that Alan was someone who knew almost nothing about the world, and that a successful joke that mentioned the Holocaust would establish that the film was going to be hilarious overall. Everyone at set loved the joke.
Lindsay Lohan was originally supposed to play Jade, but she turned down the role because she didn't like the script, and she didn't think the film would be that successful. Despite the film's huge box-office success, she went on record saying she didn't regret her decision, even though many people think it could've resuscitated her career.
Word of Saint Paul: On the DVD Commentary, Zach Galifianakis gives an explanation as to why Alan is not allowed to be within 200 feet of an elementary school - he innocently tried to play with some children which of course looked suspicious to parents. It was all just a misunderstanding.
Written-In Infirmity: Ed Helms (Stu) never grew a second incisor tooth. He had his prosthetic tooth pulled out for the movie.
Part II
On-Set Injury: Australian stuntman Scott McLean suffered critical brain damage while filming in Bangkok after a taxi he was leaning out of the window of failed to dodge another vehicle, causing his head to collide with it. Warner Bros. then issued a statement stating him was put into a medically induced coma, and were reportedly paying his hospital bills. He soon recovered, stating he had suffered "likely permanent brain and physical injuries" before suing the studio for unspecified damages.
The Other Marty: Mel Gibson was originally supposed to have a cameo as a tattoo artist, but Word of God states that this fell through after cast and crew objected to his involvementnote Supposedly Gibson responded by pointing out that they had no problem on hiringMike Tyson, a convicted rapist, for three movies. The role then went to Cooper's The A-Team co-star Liam Neeson, and the scene was filmed, until it required re-shooting with new expository dialogue after some editing. Neeson was unavailable for the reshoot due to scheduling conflicts, so the entire scene was re-shot with director Nick Cassavetes in the role.
Separated-at-Birth Casting: The casting director did a stand-up job on selecting child versions of the Wolfpack gang, Chow included, for the Flashback scene.
Throw It In!: When filming the scene where Stu discovers the tattoo on his face, Bradley Cooper genuinely broke into laughter and the director Todd Phillips decided to keep that take in the film.
You Look Familiar: Bryan Callen played Eddie in the first film, and returned as the owner of the strip club featured in Part II. Due to make up, the characters don't look the same, but sound the same.
