The phrase "misguided assumption" means a belief or idea that is incorrect or mistaken because it is not based on accurate information or evidence. Full definition
We believe this view is based on a series of outdated and misguided assumptions about the economic viability of coal, the competitiveness of alternatives and security of supply concerns. (carbontracker.org)
In view of the increasing costs of retirement, the traditional notion that retirees will only need 70 percent of their working income could very well be a dangerously misguided assumption. (fb101.com)
It is a game based on misguided assumptions. (metacritic.com)