Source: comicbook.com

1. Re-Secret Identities

The Original Story: Hey, guys, did you know that everyone knows X hero's secret identity now? Yup, it went public! Now everybody knows exactly who Spider-Man, Superman, Iron Man, Daredevil, Nightwing, Daredevil again, and The Flash are!! And that will never, ever, be reverted!

The Retcon: Here's how it was reverted. Spider-Man made a deal with the devil to bring his dying aunt back in return for having his marriage annihilated from reality. It also made his identity a secret again. Superman's got wiped away when a buncha DC Rebirth/Superman Reborn stuff happened, merging the pre-Flashpoint and New 52 Supermen. Iron Man mindwiped the world with the aid of a satellite. Daredevil just pretended he wasn't Daredevil, and everyone grudgingly went along with it.

Nightwing took advantage of the mind control tech possessed by Spyral, the spy agency he was working for. Daredevil's was wiped away for good through some Secret Wars retconning. And the Flash's was wiped from everyone's brain by the Spectre as a favor to the Flash, although it didn't really work out for the best.

And there you have it! Ten retcons that changed comic book history. Literally. This was way more fun than I thought it would be, so I might do another one. If there's demand, that is.