No word invokes more fear in a comic book reader's heart than the dreaded 'retcon'. Although plenty have managed to provide a fresh spin on a given character or premise, plenty more have done the exact opposite.
All too often associated with the reset button and convoluted backstories, writers who dabble with retroactively inserting a change into a character's past or makeup are engaging in a high-risk, high-reward game. Get it right, and you'll be praised for adding a vital and interesting dynamic to a hero, villain or story. Get it wrong, and you could just go down as the guy who tarnished a seminal comic book moment - like, say, revealing that Batman wet himself during the events of Frank Miller and David Mazzucchelli's Year One, because that actually happened!
Given most readers tend to recoil at the sheer mention of retroactive continuity, it stands to reason that these sorts of changes never go down too well. For every great change, like revealing Wolverine had bone claws before entering the Weapon X program, there are about a dozen or so more that've seemingly conspired to undermine compelling moments in DC and Marvel canon, like changing it so that Wolverine is secretly a wolf... or something.
Some changes are pretty easy to overlook, but the following retcons all left a lasting impact - and not for the better...