Who was the worst teacher you've ever had?

Eh..a guy who seemed to have a bit of a problem with expressing his thoughts verbally and a lack of talent for explaining stuff. Very unreliable, constantly cancelling classes without giving a notice beforehand, coming late...I bet he was a fine person otherwise but desperately lacking qualities that a good teacher should have.

Then some of the nastiest persons were unfortunately very good teachers. I don't know if they make you remember stuff through trauma or something but they simply make you remember it.