Maria Pepeta of Omaha, Nebraska, has a bone to pick with Marvel in the letter column of X-Men #53 (1969):
Exactly one year later, Professor Xavier was alive again.
Was this the first time a significant character was retconned back from a "for real, no kidding" death? Let's not count super-villains--they were designed to apparently die and bounce back. Is this the one that opened the floodgates?
Of course, considering some of the character assassination that's been done to Xavier's character in recent decades, Charles would have been better off staying dead...