Curse of Knowledge

A cognitive bias where someone with expertise cannot accurately remember or imagine what it was like not to have that knowledge, making expert developers poor judges of new player experience.

In games, this means

A developer who has played their game for 2,000 hours cannot accurately imagine what it feels like to open it for the first time. They navigate menus without noticing ambiguity, skip past explanations they have memorised, and fail to see what is invisible to a new player. This is why external playtesting is not optional, it is the only cure.