For information on the topic of continuity in fiction, continuity errors, and retroactive continuity (retcon), see wikipedia - Continuity (fiction)
Basic back story of AC
The basic back story of AC involves the events of the Olthoi invasion of Auberean, Asheron's involvement, The Sundering, and the conditions which summoned Isparians to Dereth. This story has been told many times, and there is often inconsistency with these tales.
Undead
In early days, the back story of the existence of undead was very different from what it is now. While today undead are almost always members of the ancient Empyrean cultures of the Dericost and Falatacot, that was not always the case. The Zone Bestiary Undead Entry originally described them much differently. Undead were said to be the reanimated corpses of fallen Isparians, possessed by evil spirits said to be linked to Bael'Zharon.
During the first story arc this was retconned to the current Dericost origin of Undead. However, there are many early dungeons and points of interest that still suggest the undead have an Isparian/Shadow source. For example, the lore of the Hilltop Ruins near Shoushi state that the undead there are fallen Sho warriors who fled from battle. Another example is the somewhat common pairing of Shadow and Undead in early dungeons.
Tanada House of Earth
The Tanada House of Fire quest involves recovering special scrolls that belonged to the houses of Air, Earth, and Water. Tanada Sajo explains that "because Aerbax succeeded in corrupting the Masters of Air, Earth, and Water, he was able to steal the scroll of each House." However, the lore of the Tanada House of Earth quest implies that the House of Earth is unaffected by the recent corruption of the House of Air.