In 1st grade I couldn't spell worth a damn, the teacher told us to write "in the future I want to be" then write what we wanted to be when we grew up. I wrote mine, handed it in, and later that day my parents got called in.

After a parent teacher conference they called me in and asked me to read what I had wrote, I read exactly what I was told to write them looked at them confused about what was wrong. The teacher calmly stood up walked to the chalkboard and wrote future, then explained that the word I had written was a bad word and I need to remember how to spell future. Later in the car I asked my parents what was the word I wrote, and they said it's a bad word. After nagging all the way home my dad snapped and said "you wrote fucker, it's a bad word and don't ever say say it".

I'm 41 now and they still bring that one up from time to time. Turns out when the teacher went to the chalkboard she was trying to avoid looking at my parents who were trying not to laugh, and that was making her laugh, so she turned to the chalkboard to get herself under control.