Fan Nickname: Madelyne Pryor has been called Maddie, Lynne, Queeny, and any number of other things by characters both in and out of universe.
Un-Canceled: The original X-Men title was canceled after seven years of horrible sales and no popularity (it was revived nine months later, but only published reprints of earlier issues). It was basically seen as a poor-man's version of the Fantastic Four. Then it was rebooted with all new characters like Nightcrawler, Storm, and Colossus, along with minor Hulk villain Wolverine. Under the skilled writing of Chris Claremont, it became Marvel's flagship title throughout the '80s and '90s.
Madelyn Pryor was originally intended to be just as she appeared — a woman who looked exactly like Jean, but nothing more. However, when plans were started to resurrect Jean to reform the Original X-Men for the 25th anniversary of the title, a major problem presented itself — Scott and Maddy had married and were happy. That's when the Summers marriage fell apart. Hence the twist that she was a clone engineered by Mister Sinister, her becoming the Goblyn Queen and her death during Inferno.
Sauron was originally supposed to be a vampire. Also, Claremont considered bringing him into the X-Men full-time as their resident doctor in the 1980s.
The Other Darrin: A few voice actors are changed, most noticeably Gambit in the final season.
Out of Order: Some episodes that were intended for season 4 were pushed up to the third season. In turn, some season 3 episodes were pushed back - most ridiculously the episode "No Mutant Is An Island" which is supposed to take place during Jean's death between the Phoenix Saga and the Dark Phoenix Saga.
Postscript Season: Word of God states the "Beyond Good and Evil" four-parter was originally intended to be the Grand Finale. A renewal order led to a few more episodes being produced before the final end with "Graduation Day."
The Canadian Wolverine and the American young!Cyclops are both played by Australian actors. Americans Xavier and Juggernaut and the Polish Magneto are all played by British actors (although in the case of Juggernaut and Xavier, their nationalities are never mentioned and they don't hide their natural accents so one could assume that the movie versions were simply from England) — which also includes the Scottish James McAvoy and the Irish Michael Fassbender. The German Nightcrawler is played by the Scottish Alan Cumming. Storm, who is from Morocco, and the Canadian Sabretooth are both played by Americans (by two different Americans, in the case of Sabretooth). The Americans Rogue and Jean Grey are played by a Canadian and a Dutch actress respectively, while the American Iceman is played by a Canadian. The Russian Colossus is also played by a Canadian but like the Xavier and Juggernaut examples above, the movie version is never given a clear nationality. Phew.
Also, William Stryker is part of the U.S. Army, but only of the three actors to portray him is American - Brian Cox is Scottish, and Josh Helman is Australian.
In First Class, Magneto is German-Irish and Xavier is Scottish (and the nationality is still unclear, as he lives his childhood in New York but graduates in Oxford). Not to mention Nick Hoult (English), and Rose Byrne (Australian) playing Americans.
Days of Future Past has three more Fake American, Omar Sy (French) as Bishop, Fan Bingbing (Chinese) as Blink and Mark Camacho (Canadian) as Richard Nixon, of all people. Plus, Mexican Adan Canto as the Brazilian Sunspot.
It's probably safe to assume that the guys playing Apocalypse in The Stinger for Days and in X-Men: Apocalypse aren't ancient Egyptians. In the former, it was Canadian actor Brendan Pedder. The latter has yet to be announced.
The CIA Agent who is the first to try and stop Shaw when he invades the CIA compound is Tony Curran, better known for playing Vincent van Gogh in Doctor Who, or being in the The League of Extraordinary Gentlemen movie, along with Jason "Azazel" Flemyng. Curran also played a knight in an episode of Primeval when Flemyng was in it, making this at least the third thing they're been in together.
Kitty Pryde has been played by three actresses, the first two being only short cameos, with the third, Ellen Page, being the first to reprise her now-major role.
Sabertooth was played by Tyler Mane, a pro wrestler, in the first movie. In X-Men Origins: Wolverine, he was played by stage and film actor Liev Schreiber, best known in front of the camera as Raymond Prentiss Shaw in the remake of The Manchurian Candidate and for the Scream movies. Made more glaring by the fact that character has a very prominent role in both films, the two actors not being made-up to look remotely alike(Schreiber's short, frizzy brown hair comparied to Tyler Mane's flowing blond.... mane), and the character's entire personality being completely different between the two films.
Alex Burton portrays Pyro in a brief cameo in the first movie, Aaron Stanford as a major role in the following two.
Gambit, played by Taylor Kitsch in XMOV, has been announced to be taken over by Channing Tatum in the forthcoming X-Men: Apocalypse.
Bryan Singer brings along cinematographer Newton Thomas Sigel, writer Michael Dougherty (and in the second, editor-composer John Ottman, who was busy during the first)... and neglects them the third by dragging them along for Superman Returns. Everyone but Dougherty is in Days of Future Past as well.
In First Class, Matthew Vaughn brought his producer/writing partner Jane Goldman, and actor Jason Flemyng (who even though of not doing make-up heavy roles after Clash of the Titans, but opened an exception to Vaughn).
Refitted For Sequel: The bathroom set in X2 was originally built for a flashback scene Cyclops discovering his powers in the first, that was left before being shot.
Singer Glenn Danzig of Misfits was the first choice for Wolverine (back when Carolco Pictures held the film rights) since he fit the character's build. After the long development hell period, Danzig had aged and it was decided to seek other options.
On the DVD commentary for the second film one of the writers mention their ideas for adapting the Dark Phoenix Saga, with Jean committing suicide by telekinetically forcing Scott to look at her and then removing his visor.