Is it ever appropriate to correct a non-native teacher's English? Four of my Japanese teachers are, as you would expect, Japanese, and they all speak English fluently. But now and then, there will of course be slip ups: I often see small grammatical errors in one sensei's handouts. They don't affect the quality of the communication, but is it appropriate to point out these errors to the sensei?

I know that if I were in Japan, I would always want for native speakers to correct me if I made a mistake; more so in the written form. However, this particular sensei has been at my uni for something like 20 years, and her English is perfect in all other regards; I wonder if it wouldn't be a little insulting to point out otherwise negligible written/spoken errors. Nonetheless, I'd like to know what some teachers think.

EDIT: When I say my sensei's English is perfect, I mean that she is able to communicate effectively, not that her speech or writing are free from grammatical errors. Like any non-native speaker who hasn't learned from a very young age, she will sometimes say or write things that sound jarring in English grammar.
It depends on how much interest she has expressed in being corrected or trying to improve and how close your relationship with her is.  Any corrections should be generally mentioned to her respectfully and in some form of one-on-one conversation.

If she has asked in class for mistakes to be pointed out, then I would not hesitate to offer the occasional correction.  Frequent corrections are probably not welcome.

If she has not expressed interest in being corrected, I would only mention corrections if the mistake could potentially lead to misunderstandings either in message tone or content.

A gray area might be if the mistake is in formal communication (e.g. grant applications) and you work closely with her.  In that case, use your own judgement.