I When that happen its needs retconing or ignoreing.
Not sure I agree with this. If something appears in a episode, it's best to figure out a way to wiggle it into continuity somehow. If necessary I would rather put somethings down to a mis-statement on the part of a character, than "retcon" it out of existence.
Some fans want to retcon way too much out of the show, simply for the purpose of supporting their own position on a personal viewpoint.