Retcon

From Superdickery

A retcon is when they say that everything that happened did not actually happen that way, if at all. Retcons confuse almost everybody if seen from the outside. However, due to the nature of retcons, if you go through a retcon you will not remember it unless you have some sort of a retcon-remembering power. Retcons are bad when written by Grant Morrison, but good when written by someone we like, Dennis O'Neil for example.

Examples of Retcons: Crisis on Infinite Earths.

Mistakenly Believed to be Retcons: House of M.

Persons With Retcon-Remembering Powers: Deadpool(?), anyone on Genosha when House of M ended (they can only remember House of M), The Spectre, Nerds.