The phrase "mistaken notion" refers to a belief or idea that is incorrect or wrong. It means you have misunderstood or misinterpreted something. Full definition
This gave rise to the somewhat mistaken notion that ~ 98 % of the genome contains «junk DNA» with no purpose or function. (massgenomics.org)
Practically every area of human knowledge and behavior may find itself faced with problems created by mistaken notions of location. (religion-online.org)