反面教師 is a cool phrase I learned during a discussion about lax discipline and Japanese parenting. The first kanji comes from hantai (opposite), the second kanji means "surface," and the last two designate those in the teaching profession.
It's read はんめんきょうし(hanmenkyoushi) and it means "someone who sets a bad example."
Now, I don't know the exact figures, but if you're a westerner in Japan, odds have got to be above 50% that you're here doing some kind of English teaching (actually, I'd love to get the statistics on that, now that I think of it). And if you're anything like me, you get the occasional question about where you're from and what you do.
I usually just say, "教師” and that suffices, but occasionally, I encounter a rarity: a Japanese person who doesn't just make random assumptions about all gaijin, and they'll ask further: "What kind of teacher are you?"
I used to try and think of the most random or unlikely thing possible. Saying "Kokugo" in my broken Japanese sometimes worked, but there was always the chance they would assume I meant MY home country's language, or that I was just stupid. "Dog grooming" was difficult to translate (犬のパームをかけ方？） and they were too liable to accept and believe I was a ビリーズ instructor without qualms. So learning this yo-ji was a fortuitous solution.
Try it out next time someone you don't know asks you what kind of teacher you are, and see what reactions you get. Mine so far have run the gamut from amused, to confused, to frightened.
And send me anecdotes about plays-on-words, or jokes that you've attempted to make in Japanese, and the results. I want to make this section a regular thing.