One of the many things that the X-Men are known for is their complicated and confusing retcons. When a franchise has as many characters and stories as the X-Men, things like origins and backstories will get lost in the weeds. You also have to factor in creative team shakeups and the pitfalls of long-form storytelling.
Ask any X-Men fan, and they can rattle off their best and worst retcons involving beloved characters like Wolverine, Magneto, Charles Xavier, and others. For those unaware, a retcon is short for "retroactive continuity," which is the act of changing a previous story after the fact. Sometimes it's meant to improve a narrative, but the results vary.
Retcons can drastically alter what fans know and love about their favorite heroes and villains. The X-Men have been involved

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