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.
Trivia: Longtime X-Men writer Chris Claremont is shown (guy with a lawnmower) in the Grey's neighborhood at the beginning of the film.
Trivia: This is the first time where all the original Uncanny X-Men are featured in one film - Prof X, Cyclops, Marvel Girl (Jean Grey), Beast, Iceman and Angel.
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: 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 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: It is revealed in the commentary that the man in the vegetative state that Moira McTaggert takes care of is actually meant to be the twin brother of Charles Xavier, Patrick. In the comics, Xavier has a twin sister, but this change was made to make things more interesting.
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.
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: Avalanche (from X-Men Evolution) makes 2 small cameos. The first being the mutant cure speech from Magneto in the forest (standing by Juggernaut and Quill), the second being during The Battle of Alcatraz.