This is not a ‘worst teacher’ but a worst situation as there was literally nothing that could be done to help, and the uni set them up to fail.
I had an international PGCE student in my reception class as a trainee teacher.
Unfortunately she had an extremely, extremely thick accent to the point where I couldn’t understand her. I could just about do in a quiet class with really focussed listening and lots of repetition, but the children couldn’t. We didn’t have a successful obs because she couldn’t communicate. They would outwardly tell her ‘what?’ and cry because they didn’t understand her. I don’t know how she passed the interview. 
I contacted the university immediately and was immediately fobbed off and insinuated that I was some kind of racist
When they came in at my insistence, they immediately saw what they problem was but felt it MY responsibility to give her feedback such as ‘enunciate words’ ‘speak clearly’ ‘ensure pronunciation reflects standard English’. She couldn’t teach phonics, because she couldn’t say the sounds. Obviously it was how the woman spoke and she was new to the country, she wasn’t going to develop anything vaguely neutral within a term.
I left during that term and she joined my partner teacher’s class. The university just said that ‘she would learn’.
I think of her sometimes- a shit situation all round and I felt so sorry she was in that position. She must have felt really uncomfortable and me + my TAs felt like utter dicks asking her to repeat constantly.
Teacher shortage, I suppose…