Shortly before Gwen Stacy died, she moved to Europe to live with relatives after her dad was killed. Several decades later they revealed that while she was there, she fucked Norman Osborn. Note that of course she never fucked Peter. Also she had two goblin babies with Norman that grew at an accelerated rate so they could be significant characters and not just toddlers in the present time. At least they have the decency to almost never reference this story in later issues.
Obviously the biggest one among many in the last few years is saying that Captain America was a Nazi all along. Pretty strange considering we had been literally reading his thoughts for 80 years and never seen anything close to that. Iceman being gay is another bad one partially for the same reason. At least there you can point out that Jean Grey obviously used her psychic powers to turn him gay.
There's that time Kevin Smith retconned that, during the famous scene in Batman: Year One when he crashes into Falcone's mansion for the first time, his first real big appearance as Batman, he actually peed his pants. I don't think it's nearly as bad as other examples, because it's at least very minor, and doesn't change the character going forward very much, but it is funny and notable.