Trivia: The combat simulation the X-Men are running on their first appearance in the film features the dread Sentinel robots. Originally, these were meant to appear in the second movie already, but the idea was scrapped. The simulation also bears a close reference to the parallel comic storyline and later movie X-Men: Days of Future Past, where the Sentinels rebelled against humanity, killed most of Earth's superheroes and erected a totalitarian regime in the US.
Trivia: Juggernaut says "Don't you know who I am? I'm the Juggernaut, bitch." That was taken from the Juggernaut fan made episode found on the net. Try searching google for it.
Trivia: Stan Lee makes a cameo appearance in the beginning of the movie when Jean is being recruited and showing off her powers. He is the man holding a hosepipe. (00:02:50)
Trivia: Not exactly a mistake since his name is never spoken, but according to the writers, the porcupine-like character was incorrectly named "Kid Omega" in the credits, "Quill" is the corresponding comic book character.
Trivia: Matthew Vaughn was originally slated to direct, but dropped out of the project because of the rushed schedule, feeling that he couldn't make a movie up to the standard of the first two with the time allotted. Several of his decisions did make it to the final film, though, including the casting of Kelsey Grammer. Vaughn later went on to direct the prequel "X-Men: First Class."
Trivia: Ellen Page alleged that director Brett Ratner was inappropriate on set, including outing her as a lesbian at a cast event eight years before she actually came out publicly, and suggesting that another woman should have sex with her to make her "realise she's gay." Co-star Anna Paquin backed up her claims.
Trivia: Bryan Singer's original vision in the film involved Jean coming back to life as the Dark Phoenix and being manipulated into joining the Hellfire Club, led by Emma Frost. The film would be about the struggle between the X-Men, the Hellfire Club and the Brotherhood, who all seek to control Jean for their own purposes. In the end, Jean would have killed herself to save the X-Men, and transcended a higher plane of existence, which one of the writers compared to the ending of "2001: A Space Odyssey."
Trivia: Brian Singer, who directed the first two films, wanted to make this third film, but he was unable to due to his commitment to "Superman Returns." He reportedly had mixed feelings about the finished film, expressing dissatisfaction over the handling of the Phoenix saga, but enjoying the "cure" storyline and the character of "Leech." However, his dislike of the Phoenix saga lead to his retcon work in the films "First Class" (which he produced and co-wrote) and "Days of Future Past", which he directed.