The 20′ By 20′ Room: Oh No You Didn’t
Rob at the 20×20 room talks about Retconning, the practice of changing past events for continuity’s sake.

I’ve had to retcon. Nearly every GM has had to do so at one point or another. Names are forgotten and misremembered, places, people, the works. Plot threads long since forgotten have to be reinterpreted in the light of new events.
One small example of retcon occurred recently in Strange Bedfellows. An NPC child of Deirdre, Percy, was originally only passingly familiar with Corwin’s daughter from his universe, Alais. The original documents that I sent to the player, Ginger, stated that Percy had not met Alais that much.
In a game post, talking about her family, Alais made a passing reference that Percy had met her mother, the Queen of a shadow adjoining Paris.
I liked the idea of a stronger relationship between Percy and Alais, and wrote in a subsequent post that Percy had visited her realm a couple of times, and is somewhat familiar with it.
So, I retconned the past, and revised it for the present.