Trivia: In the beginning when the 20th century fox logo is fading the 'X' stays on just a bit longer.
Trivia: On the X-Men 2 DVD, the filmmakers mentioned this. If you think about it, in X-Men (only really relevant to the first film, given Nightcrawler and Deathstrike turn up in the second one), all of the X-Men are beautiful and handsome actors and actresses, but all the villians are weird-looking, ugly beasts, or freaks of nature. Magneto's the one exception.
Trivia: Although this is not a mistake, I just want to point it out. When Toad (Ray Park) is fighting Storm (Halle Berry) at the statue of liberty he kicks her into an elevator shaft. He then proceeds to remove a bar that was holding the doors of the elevator open and he does a spin move that is a definitely supposed to look exactly like the spin he did with his lightsabre in Star Wars: Episode I - The Phantom Menace. For those of you who don't know, Park played Darth Maul in that film. [Cross reference with Sleepy Hollow, also starring Ray Park as one of the swordsman actors - he does a similar move in that too. Clearly a trademark.]
Trivia: This comes from one of the set designers, and is referenced in Wizard: the Guide to Comics; If you look closely, the designs of the sets often have "hidden" X's and O's incorporated into their designs.
Trivia: Joss Whedon did a rewrite for the script, but only two lines were kept, uncredited. One is "do you know what happens to a toad when it's struck by lightning? The same thing that happens to everything else", which he maintains was meant to be much more throwaway, but was instead botched by Halle Berry's dramatic delivery. The second, much better line is Wolverine's "you're a dick" directed at Cyclops to prove it's the real him.
Trivia: Dougray Scott was cast as Wolverine, but had to pull out due to filming of Mission: Impossible 2 overrunning. Russell Crowe was then asked. He also turned it down, but recommended his friend Hugh Jackman instead. Crowe turned down the role because his part in Gladiator was connected with wolves (more strongly during filming than in the final cut) and believing Wolverine was also based on a wolf, he didn't want to be typecast as "Mr. Wolfman" as he put it. Hugh Jackman also thought Wolverine was wolf-based, and did a couple of weeks of research on wolves. When he brought that to rehearsal director Bryan Singer corrected him. Jackman didn't initially believe wolverines were real animals, and Singer told him to go to the zoo.
Trivia: The man who plays the guard in the plastic prison cell is David Hayter, the man who wrote the screenplay and, to most gamers, the voice of Solid Snake in the Metal Gear Solid game series.
Trivia: In the comics, Mystique in her true form wears a narrow white gown. The reason she is practically nude in the films was because it made it easier for the special effects team to morph her into other characters. Rebecca Romijn has said that wearing nothing but prosthetics and blue body paint was liberating.