I knew a couple of people who couldn't jam or play cover tunes. Nor could they really get people to easily understand their original music. I taught them some fundamentals, and it castrated them. They went from writing/producing stuff i'd never heard and felt was different, to crap. (I feel really bad about it to this day). I'll never teach someone who has 5 or 10 years of self taught and written songs again. I feel i did them a disservice.