When you have a misapprehension, you make a mistake trying to understand something, which leads you to believe something that isn't true. You may have a misapprehension about your brother's motives for being so nice to you this week.
You may have an uncle who is famous for his misapprehension of luck and probability, which must be why he’s lost so much money gambling. He also has a misapprehension of geography, so he thinks he can get from his house in Maryland to your house in New Hampshire in under four hours — by bus! No one ever said your uncle was stupid — he just has a misapprehension of some basic things that the rest of the family gets.