Interesting post on TeamLiquid about the storyline to Starcraft 2 - Heart of the Swarm. While I have come to generally not care, and in some cases embrace retconing facts, what is described here is something that you CAN'T retcon, and that's character personality and motivations.

Since the characters are what people are connected and invest in, if you retcon them, then you end up with an emotional disconnect. Nobody should be emotionally connected to a random fact or tidbit about a universe, but everybody should be connected to the characters.

Spoilers in the link obviously.