In its 81 year history, Marvel comics has made more than few creative blunders. They have allowed prominent creators a level of control that would ultimately prove detrimental to the characters they had been entrusted with. In other instances they have relied upon shock value to increase sales and draw readers back to a flagging title, usually by "killing" a significant character, bringing another back from the dead or replacing the face behind the mask.
Regardless of fan reaction to a given creative choice, the on-going narrative of superhero comics will mean that such choices will need to be undone. In these instances, the publisher is left with no choice but to "retcon" the situation. This is basically an industry term for an often clumsy "Deus-Ex Machina" that resets the pieces on the board and allows new stories to be written from a relatively common baseline.
In some cases though, if the change is small enough or niche enough then Marvel go with the other tried and tested formula of "sweeping it under the rug" and acting as if it didn't happen. Which leads to fascinating oddities that manage to be both in and out of continuity.
It is some of these "Schrodinger's Plot-Threads" that need to be talked about now...
10. Legion Is His Own Father
Legion is the mutant son of Professor Charles Xavier and Gabrielle Haller. He suffers from severe mental illness, including dissociative personality disorder.
Holding Magneto responsible for the failure of his father'dream, Legion uses his mutant powers to travel back in time and kill the Master of Magnetism BEFORE he becomes his fathers greatest enemy. Only to end up killing Xavier instead and ushering in the fabled "Age Of Apocalypse".
As far as Marvel is concerned this is pretty much the events of the "Legion Quest" storyline. However, those who have read the book know this is not the case. Legion returns to Israel where a young Erik Lehnsherr and Charles Xavier first meet through a mutual friend Gabrielle Haller who is a patient at the hospital where both men work.
And here is the moment where the wheels come off.
While Xavier and Lensherr are out for the evening, Legion proceeds to take on his fathers' form and seduce his mother. As the two begin to kiss, just off the panel, we cut away to the next part of the story. Only for a group of time-displaced X-Men, Magneto and Xavier to confront Legion, who is found standing over the ravaged body of Gabrielle Haller.
The rest is pure mathematics. David Haller is not more than twenty years old at the beginning of the "Legion Quest" storyline, and the primary struggle young Charles Xavier is having is whether or not he should act upon his feelings for his patient Gabrielle Haller. The off-panel assault is never mentioned again but is never explicitly retconned either.
If the original plan was to reveal that Haller was his own father, then it was one which was never followed up on, if not then what was the point of the sequence as it has no bearing on the characters or the "Age of Apocalypse" storyline that it kickstarted?