At one point, the current antagonist triggers an event where he takes control of what is essentially an end-game raid boss and an endless horde of undead to destroy the entire light-side faction. The developers basically put a game-over button in their MMO for a player to find, and the situation would have been completely unwinnable if the MC didn't have future knowledge.
The Altars, which are introduced as a way to both sabotage Dungeons from the inside as well as acquire incredibly powerful monsters, are ignored after the first 6 out of 72. The World Tree Tea that provides a massive boost to his Intelligence stat is sort of forgotten after a couple of months.
Supposedly, this is the work of the servants of the Demon King, but common knowledge can't simply be erased from the collective consciousness over the course of a single generation. The people that grew up being taught this stuff would still be alive.
For example, the MC's teacher is described as someone who could "disintegrate the viceroy's castle with a fingertip, " but when they assault a city they sort of forget she can do that and go with a frontal assault instead. And then she passes out after casting a single lightning bolt.