When I was 30 I learned that I had pronounced and spelled the German word "unbedingt" wrong my entire life. I thought it was "umbedigt" as in "um jeden Preis". I thought all others spelled and pronounced it wrong or spoke more elaborate than I.
Had an argument with my ex once (both speaking german as a third language) about the pronounciation of 'Umgebung', where she made it sound like "Um'g-bung" for some reason. Ended up asking a random train conductor to settle it.