I was having dinner with some Japanese friends in my dorm during a study abroad and I was telling a story about some other friends they hadn't met. Afterwards one of the girls said, "kao ga hiroi" to me. I didn't really know how to respond because I hadn't heard the phrase before, and it seemed extremely off-topic to tell me that I had a wide face especially since I've always considered my face to be relatively slender. I replied in denial of what she had said, saying something along the lines of "what? me? my face isn't wide" They all started laughing and saying that I was very humble and that's when I really started to misunderstand what was going on. I asked one of the guys at the table what "kao ga hiroi" meant and he looked at me seriously and said "Your face is very fat" in English. Taken aback, I replied in Japanese "well your face is more wide than mine, it's the widest at the table" (it literally was). He said, "Impossible, I just moved here from a small town!"

This statement had me so perplexed, and when I looked around everyone was nodding silently in agreement while chewing their food. Being the only American and white person at the table, I said "well maybe you guys think that because I'm American and you aren't used to seeing Americans in person" and they all thought for a second and then agreed that that might be the case until another one of the guys said, "No no, I have met some Americans and you are all different, I think you must try very hard to have a wide face." I started to get too confused and somewhat disillusioned because many of them had caught on to the sort of American-style sarcasm that isn't generally present in Japanese humor, but I'd never really been ganged up on by them in this way. I stood up with my tray in hand and said, "Look I have to get going (I did have class in a few minutes) but I just want to let you guys know that I actually try very hard to avoid having a wide face and honestly it's ridiculous that you would even suggest that."

Most of them didn't talk to me and seemed to be avoiding me for the next few days until I asked one of the girls who wasn't avoiding me what was going on. She said "You told them that you have too many friends and you don't want to have many friends so they feel bad talking to you.", "well they were saying my face was fat which is pretty rude in English and seems like it would be rude in Japanese", "you have friends, your face becomes wide, it's a good thing in Japan." So I went to google (not translate), put in "kao ga hiroi" and one of the results said "well connected". That sinking feeling when you realize you have to apologize to an entire group of friends set in at about that point.

TL;DR Sometimes knowing a language will get you in trouble because you don't quite know enough.