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.Daniel4646
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: 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.