And there's no need to assume there's some sort of "messup" involved. I feel this needs to be said again: creators don't care about the aftermarket.
Their only concern is "is this a good story, yes/no, and will it sell copies, yes/no?"
I doubt the scheduling of Gambit's appearances even entered Claremont's...or anyone's...mind. He and the Simonsons plotted out DOFP, included Gambit because the story takes place after the story in 265-267, and that was that.
No error, no screwup. Nothing "came out too early", there was no "mixup at the distributor", etc. This isn't the first time such an "out of chronology" story has happened...in fact, it happened all the time ("The events in this story take place BEFORE/AFTER thus and such")...it just happened that THIS time involved the debut of one of the more popular characters created in the time period.
I have to wonder why we were told to pull them off the shelves, then.
-slym