A Retcon (Retroactive continuity adjustment) is the alteration of previously established facts within the continuity of a fictional work. EGS is pretty good in this regard, with lot of things obviously planed much sooner than when they appeared. Still, there ARE some cases where Dan decided that it's better to break continuity, and there are others where he thinks about it. One remarkable point is that he is not trying to hide those cases from the readers: in fact, he points them out.