Speaking Japanese
When speaking Japanese, non-native speakers need to remember to be polite.
Most languages have at last two levels of speech. In general, they are formal and informal. In the US, this is especially true in business. You call people Mister, unless told otherwise. You speak and act politely, unless you become very familiar with your superior. Do you slap you boss's back and tell him "Good job, dude"?
In Japanese, the line is even more pronounced. Unfortunately for most Japanese learners, a Japanese speaker will not correct a non-native speaker when they speak informally. Many Americans will come back from Japan thinking their Japanese is all that. I certainly have many students like that as well. And for the most part, their confidence is well founded. Their Japanese is relatively fluent and unobstructed by the fear of using the wrong word.
However, if they are too informal with me, I will always correct them. I don't mean to be a hard-ass, but someone needs to correct them because if and when they return to Japan for work or graduate study, they cannot talk informally when talking to a business colleague or professor. They have to learn to turn the formality switch on and off in any given situation. And the level familiarity rarely has anything to do with it. I worked at a Research Center for two years in Japan and became very familiar with my bucho (division chief). We often drank together, and he is the one who dubbed me the "American who speaks English". But one night while drinking, I spoke to him a bit too familiarly. Now, in Japan, drinking often excuses an error in judgment, and most will laugh it off the next day. But my error in being too familiar with my bucho put me in his doghouse for two weeks. He literally did not speak to me during that time, relaying messages to me through others.
The bottom line is--been there, done that. So I tell my students to speak to me formally whenever they decide to speak to me in Japanese. If they think I am a hardcase, then so be it. I take it upon myself to be their practice partner, their opportunity to learn how to turn that formality switch on and off.