Back when I was in college, my Spanish teacher thought it was his job to enlighten us about news of which we should be socially conscious (rather than simply teach us how to speak Spanish). This was around the time that every bleeding heart rock singer and his brother were using the situation in South Africa to advance their media exposure. It was also while Nelson Mandela was still in jail.
This teacher asked the class if they knew what the problem was called and I raised my hand and said "apartheid" pronouncing it correctly and he reiterated my reply by saying "a-par-THede". He clearly was pronouncing it incorrectly but was not so subtly attempting to "correct" me.
Among the things I've overheard many times while working as a teacher is other teachers offering up corrections which are incorrect, giving out incorrect information, serving up dubious theories, and making serious grammatical errors themselves. My husband says he overhears a fair amount of such behavior in his work as well, particularly from certain specific teachers.
One of my former coworkers (who was a total nightmare) had the "dog" and "cat" theory of personality which he felt students needed to know and understand. I never really sat through the whole lecture but multiple exposure to snippets of his bizarre theory indicated that bad people were supposed to have the personality of dogs and good people that of cats (or vice versa). I can only imagine what the students thought of this notion and what it said about western people if they felt it necessary to reverse-anthropomorphize themselves in such a fashion.
Even when the students know the teacher is wrong (as was the case with my Spanish teacher and myself) or full of baloney (as was the case with the "dog" and "cat" guy), they don't say anything. In some cases, they resist out of uncertainty. In others, they don't want to anger the teacher out of fear about how it'll affect their grade or rapport with the teacher. I'm guessing in some cases it's also about being polite.
To be honest, I occasionally misspeak and make a grammatical error (as I'm sure everyone does) but I always correct myself. I think some people are embarrassed to acknowledge a slip of the tongue with self-correction and some people aren't aware that they're making mistakes. Teachers aren't perfect, no matter how educated they are or how hard they try. Still, it's hard not to cringe when you hear someone reinforcing a mistake with students or making their correct English incorrect.