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.

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.

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.

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.

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.

All the flashing images in the Marvel logo at the beginning (you need to freeze-frame to see them properly) are from the X-men comics and have at least some relevance to this movie's plot. Submitted by Blibbetyblip

Longtime X-Men writer Chris Claremont is shown (guy with a lawnmower) in the Grey's neighborhood at the beginning of the film.

After the credits roll out, there's an extra scene about Professor Charles Xavier.