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."
Magneto is very muscular in the comics, but the slender Ian McKellen was cast because he's one of the finest actors on the planet.
Hugh Jackman is a foot taller than Wolverine in the comics, but his performance has won over the fandom, and it's now difficult to imagine someone else replacing him.
X-Men: Days of Future Past: In the comic books, Bolivar Trask is a taller fellow while Peter Dinklage, who portrays him, has dwarfism. He's an Emmy-winning actor whose performances are frequently met with critical acclaim, and since appearance has very little bearing on the character, nobody has made anything of it.
Fake American: The Dutch Famke Janssen as Jean Grey, the New Zealand Anna Paquin as Rogue, the Canadian Shawn Ashmore as Iceman, the English Nicholas Hoult as Beast, the Scottish Brian Cox as William Stryker and the Canadian Ellen Page as Kitty Pryde.
Fake Nationality: The Canadian Wolverine is played by the Australian Hugh Jackman, and the German Nightcrawler is played by the Scottish Alan Cumming.
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).