"Mistaken idea" refers to a belief or thought that is incorrect or wrong. It means someone has misunderstood or has gotten the wrong impression about something. Full definition
Last but not least, some women hold mistaken ideas about dating without realizing it. (feedspot.com)
In the crazy humdrum of online job search and application, its easy to mistake the idea of a resume as a 21st century concept invented well after the word processor and the Internet. (tapwage.com)