My worst teacher was my sixth grade politics teacher. We didn't learn a thing about politics and he was constantly reciting a line from the German basic law, "Die Würde des Menschen ist unantastbar." (Human dignity is inviolable). He would say that line whenever he found a chance, appropriate or not appropriate. I was always wondering how he managed to become a teacher in the first place.
He also didn't really take a look at our homework, you could show him anything and he thought it was the homework and checked it.