It wasn't so much the spelling I got slaughtered on, I've always been pretty good at that. It was punctuation and Grammar.
Just the thought of that letter coming back proof read (through my boss of course) turns my face red to this day. That was more than 20 years ago. Not only did I learn that I need to step up my game when it comes to written communication, I learned that NO official communication leaves the office without at least 2 proof reads (By co workers) and of course the boss signing off on it.