When filming the scene where Stu discovers the tattoo on his face, Bradley Cooper genuinely broke into laughter, and co-Writer and Director Todd Phillips decided to keep that take in the film.
Ed Helms (Stu) was severely sick from food poisoning when dancing in the street with no shirt on. After each take Helms would curl into a fetal position until it was time to film again.
In the original script, Alan was meant to have his beard shaved instead of his head. Zach Galifianakis refused.
Teddy's achievements closely resemble that of Ken Jeong, who took part on the High IQ team, played violin in the orchestra, was elected to student council, and graduated at the age of sixteen.
Former President Bill Clinton visited the set while on-location in Bangkok, Thailand. Some news sources erroneously stated that he had filmed a cameo.
Co-Writer and Director Todd Phillips, Zach Galifianakis, Bradley Cooper, and Ed Helms were all promised a percentage deal for the film on top of their paychecks.
Bryan Callen, who plays the club owner in this installment, played the chapel director in the previous film.
Mel Gibson was supposed to play the tattoo artist, but the idea was later dropped following protests from the cast and crew. He was replaced by Liam Neeson. Co-Writer and Director Todd Phillips later wanted to re-shoot Neeson's scene, but Neeson was unavailable to return, as he was filming Wrath of the Titans (2012). The scene was subsequently re-shot with Nick Cassavetes in the role.
Todd Phillips realized the potential of Mr. Chow (Ken Jeong), and decided to give him a bigger role in this movie.
Bradley Cooper's father was in the last throes of cancer during the filming of this movie, and died three days after Cooper returned home from the shoot.
The three Billy Joel references in the movie (a "Glass Houses" poster in Alan's room; the song "The Downeaster 'Alexa'" on the soundtrack, and the parody of "Allentown" sung by Ed Helms) stemmed from Todd Phillips' desire to decorate Alan's bedroom with the poster, feeling that Alan would strongly identify with it.
Stuntman Scott McLean was seriously injured in a traffic collision while filming a stunt taxi chase scene near Bangkok, at approximately 3 a.m. on December 17, 2010 (local time). He was doubling for Ed Helms when his head smashed into a truck, leaving him with a huge gash over the right side of his head and flesh torn from his skull. His head suffered critical brain damage and he was put into a medically induced coma. Just a month after the accident, he came out of his coma. The shots of McLean in the moments before the crash were actually used in the final cut of the film.
As the Wolf Pack enter the Buddhist monastery, Zach Galafinakis is seen performing a Christian cross, he actually performed a Greek Orthodox sign of the cross consistent with his Greek origin and faith.
The only entry in The Hangover trilogy not to feature "In the Air Tonight" by Phil Collins.
Entering the Buddhist monastery, Allan makes the sign of the cross on the Eastern Orthodox way.
The posters of famous wrestlers in Alan's room are actually enlarged images of WWF trading cards. The white border around the photos and the yellow "classic WWF" logo that was only used on those trading cards is visible in several shots and a sleeve of the regularly-sized trading cards is visible next to Alan when Alan asks Stu if Stu is really serious about inviting him to the wedding.
Crystal the Monkey appeared on Malcolm in the Middle (2000) season three, episode twenty-two, "Monkey".
The photo during the credits of Phil holding a gun to Mr. Chow's head resembles the famous Eddie Adams Vietnam War photo of a Viet Cong being executed in the streets of Saigon.