Retcons/reveals that establish someone wasn't guilty of what we thought they were! Retcons/reveals that establish someone has been guilty of X for a long time, but now we're just finding out!
Hal Jordan wasn't a murdery reality-changing drunk driving egomaniac, he had the yellow fear demon in him. Hawkman, et al, being happy to brainwash or magically de-competency anyone who puts them at risk, from Dr Light to Batman.
Which ones work? Which fall flat for you? Which, just piss you off?