B-Team Sequel: Neither Bryan Singer (who directed the first two X-Men films) or Brett Ratner (who directed the third one) were involved in the production of the film, although both were interested in returning to the franchise.
Deleted Role: A young Storm, played by April Elleston-Enahoro, originally appeared in a couple scenes of the film, but it was all cut and relegated to the DVD. She can be still seen in a Missing Trailer Scene.
Disowned Adaptation: Rob Liefeld says he and Jim Lee went to watch the film just to see the portrayal of Deadpool (which Liefeld created) and Gambit (which Lee created) and both of them were unhappy with what the movie did to their characters and Liefeld says he wrote a note to one of Fox's executives and wanted to reshoot the film with Deadpool having a mask but Fox executives unsurprisingly scoffed him off.
Dyeing for Your Art: Liev Schreiber gained a lot of muscle to play Sabretooth, enough so that he had to buy new clothes because his regular wardrobe wouldn't fit anymore.
The Fox executives decided that it would be a brilliant idea to take Deadpool, the Merc with the Mouth and possibly one of the most popular characters appearing in the movie, introduce him and then remove him quickly, and then sew his mouth shut. Because he would be too entertaining and take attention away from Wolverine or something (no-one knows the exact reason).
Director Gavin Hood and top Fox executive Tom Rothman reportedly had clashes over the film's creative direction. One infamous incident happened while Hood was off-set, at which point Rothman took it upon himself to have one of the sets repainted from Hood's original Darker and Edgier theme into something more Lighter and Softer.
Hood has also stated that the scripting process was a mess, and that portions of the screenplay were hastily rewritten as the movie was filming.
Fan Nickname: The version of Deadpool appearing in the film has gained a few, mostly related to how he is really nothing like the Deadpool from the comics. The most popular one appears to be DINO (DeadpoolIn Name Only). He is also referred to as "Dudepeel" by Imageboards, "Cesspool", "Barakapool" (because of the swords in the arms) and probably others.
Franchise Killer: The film was a brief Franchise Killer for the X-Men movies, since it was intended to take the franchise in a different direction following the original trilogy, but the terrible reaction to it killed these plans and a different (and much more successful) direction was chosen in the semi-reboot X-Men: First Class.
Old Shame: Hugh Jackman isn't proud of the film, as he felt it did little justice to his character despite the high praise he received for his performance in an otherwise underwhelming movie; he helped make up for it with The Wolverine and Logan.
Tyler Mane played Sabretooth in X-Men. He was replaced by Liev Schreiber for this movie. A tie-in comic had to be released to explain why was he supposed to look so different between the films (apparently, he was mindwiped and experimented on to make him bigger and more feral).
Promoted Fanboy: Will.i.am was a fan of the X-Men franchise who identified Nightcrawler as his favorite character, and was subsequently cast to play a mutant with similar teleporting powers.
Similarly, Ryan Reynolds was a big fan of Deadpool, and his disappointment in this film's handling of the character led him to campaign for years for a better treatment of him on the big screen. To the joy of all, he succeeded gloriously.
Stillborn Franchise: Played straight, but eventually subverted. This movie was intended to be the first installment of an X-Men Origins film series spun off from Fox's X-Men Film Series, to be followed by X-Men Origins: Magneto, X-Men Origins: Deadpool, and X-Men Origins: Gambit. After disappointing reviews for Wolverine, the Magneto script was eventually incorporated into the much better-received X-Men: First Class and the X-Men Origins series was scrapped. Years later, Fox would put resources into making Deadpool (2016) and Gambit movies (although after countless delays and Fox's merger with Disney, the Gambit movie was finally shelved in May of 2019), but without any actual connection to this film.
Troubled Production: Filming was delayed by weather and Hugh Jackman's commitments with Australia, begun with an unfinished screenplay that was rushed due to the then upcoming 2007-2008 writer's strike - director Gavin Hood detailed that during shooting in Australia, script pages would be sent from LA, at times in the night prior to them being filmed; also, characters such as Gambit and Deadpool were late additions, explaining why the former appears so little and the latter wound up changed to an unfaithful version - and saw Hood entering conflicts with Fox executive and overall bean counter Tom Rothman micromanaging every aspect, right down to repainting sets without telling Hood. Even the movie's title was changed without Hood being informed, with Jackman telling him once he saw it on IMDB. Plus, the second unit didn't match Hood's style, making him feel the action scenes were like ones in an 80s flick. Eventually Richard Donner (husband of producer Lauren Shuler Donner) was forced to join production in order to mellow out things. And an incomplete version with missing special effects leaked online one month before the film's premiere. The film could have ridden out the bad critical and fan reception like the previous film, if not for the disappointing box office. This caused the series to get a partial reboot with X-Men: First Class.
Writing by the Seat of Your Pants: Gavin Hood has stated that the scripting process was a mess, and that portions of the screenplay were hastily rewritten as the movie was filming.
The Other Darrin: Ryan Reynolds was going to voice Wade Wilson in the video game but because of his unhappiness with the film, Steve Blum voiced Wade.
Role Reprise: When the game adaptations were released, only three actors from the film reprised their roles: Hugh Jackman, Liev Schreiber and Will.i.am. The other characters have had their designs retweaked to not be exact carbon copies of their actors; Gambit barely looks like Taylor Kitsch, for instance.