I had a great guitar teacher when I first started out. He was actually teaching my friend guitar for years, and came highly recommended. (I suggest when looking for a teacher to meet with them first before signing up, and it helps if they come with references.)
I used to teach guitar lessons (back when I had time). Put up some fliers in my neighborhood, and soon had 2 students. Then by word of mouth only, was up to 7 students. Then disaster struck, and someone said they heard I taught guitar lessons, and wondered if I could teach bass lessons! (I was playing bass in a band at the time, and had a lot of experience on the instrument, so I felt qualified.)
Unfortunately, the student-to-be was a teenager. He didn't know any music theory, or know anything about the bass. He wanted to play metal (and with a pick). I said "ok, but first let's learn the instrument" and proceeded to show him theory (he wasn't interest in the least) including reading music (next!) and showed him how to position his fingers, pluck the strings with his finger (wasn't interested, wanted to use a pick) etc etc. He made reading his face much easier than reading music - b/c he was bored and just wanted me to teach him how to play metal - not any of this other boring stuff he wasn't interested in. [note - I had no objection to playing with a pick, but had first wanted him to learn left hand techniques before working on coordinating the right hand with pick playing.] Looking back, if I had showed him some metal songs that just thumped on the E string, he would have been happy as anything, and learned nothing.
In the end, he quit after 2 lessons (painful for both of us!), and I never taught a bass lesson again!
So my advice to avoid future "bad teacher" stories is to be realistic with your expectations! That said - a lot of the above stories show the result of an industry that doesn't require any credentials to set up shop and teach lessons. There are some bad teachers out there.
