This is my (late) husband R’s story which he used to laugh about.
One day he was running a temperature. His red face drew the Principal’s wife attention. She asked him, “why is your face all red?”
R in those days wasn’t at his best in the English language, and his reply was, “I am in a temper”. He meant he wasn’t well, and was having a temperature.
Poor lady! She was taken aback by his answer. She demanded, “young man, now why are you in a temper?”
Time to confess: tell us about a time when you used a word whose meaning you didn’t actually know (or were very wrong about, in retrospect).