Retcon means "retroactive continuity" and means to alter previously established canon, either by ignoring the previous material or working in additional details to completely alter the perspective upon that original material. Retcon through ignoring others' writing is considered uncooperative writing and not generally advocated, while working within existing material to expand it, such as a Character's backstory, is commonly done. Major retcons on the Never-ending Story include the Star Wars Retcon.
The Never-ending Story
Although not retconning on the grandiose scale, Ares the Writer had a habit of ignoring and, thereby, retconning continuity during the earliest posts of the Never-ending Story1. He wrote out the Character DarkSide, later reinstating the Character for use, and negated posts by Galvatron the Writer. His habit of ignoring and negating posts became a running meme for the Character later[NeS1 1], to the point of personality trait, meaning Ares constantly commits minor feats of retcon on a regular basis, claiming that it never happened (CITATION needed).