This question isn't just about bad retcons, it's specifically about unnecessary retcons. Retcons of things that pretty much bothered no one except the writer making the retcon and maybe 3 fans.
4 I can think of:
Nicieza's Psylocke/Kwannon/Betsy Braddock retcon. Was anyone really calling for this story out in fandom? Did it clarify, improve or simplify anything about the original Asian Psylocke story.
Norman Osborn and Sandman are related from Byrne's Chapter One. Okay, so since Norman and Sandman both have that weird wavy Osborne hair, this bothers Byrne so much he has to have an in-story explanation for it?
Loeb's Shatterstar/Benjamin Russell storyline in X-Force. This is the first time I realized Jeph Loeb was a horrible writer. It was also the first time in an X-bok where I even saw the editors in the letters page admit to the readers that they had no idea what the story was about and couldn't make sense of it either! The senselessness of the story line was even addressed in-story later on in the year in an annual written by Nicieza.
Magneto' real name: I heard there was some story about Magneto's real name being re-retconned back into Magnus. Magneto goes and kills the guy who told him his name was Lensherr. Years later I think it was re-re-retconned back into Lensherr. What a waste of ink.