First Class changing Xavier 's age when he met Erik is a retcon because it's literally "retroactive continuity", overriding a previously established bit of information with new information.
As far as an example of an actual continuity error goes, here's one from the Harry Potter novels:
In the very first book, Harry Potter and the Philosopher's Stone (Sorcerer's Stone in America), we are introduced to a minor character named Marcus Flint, who is a Sixth Year; the character is also featured in the second book, Harry Potter and the Chamber of Secrets, where he is in his Seventh and final year, and again in the third novel, Harry Potter and the Prisoner of Azkaban, even though he should have graduated from Hogwarts at the conclusion of the second book.
Rowling forgot that Flint was supposed to be gone from the school when she wrote PoA, creating discontinuity between that novel and CoS, and never bothered to try and explain, within the course of the narrative of PoA, why he was still at the school during that book.