I’ve got a few, one that happened to me and some that happened to people I know. Japanese has great potential for mistakes because there are relatively few sounds compared to other languages. Even native speakers sketch kanji in the air for words that sound alike if they want to avoid confusion.
Along with words that sound exactly alike there are plenty of words that differ by a single vowel, or even worse, differ only in the length of the vowel sound or a change in pitch. English doesn’t pay any attention to duration or pitch so native English speakers screw up the words that depend on these differences quite often. Most of the time it doesn’t matter since you can get it from context. The difference between the edge of a cliff, bridge, and chopsticks is pretty easy to figure out, for example.
I was with a Japanese friend of mine late one night when he called his mother. It was an abnormally short conversation. I said, “Okashita no?”
He was shocked and angry, and was very close to yelling at me when he said, “Why would you say something like that?! What’s wrong with you.”
I should have said Okoshita no? which means, “Did you wake [her] up?” Okashita no? means “Did you rape [her]?”
I’d never even heard that word before so I had no idea what I’d accidentally said until we sorted things out.
My friend had been in Japan for about a year or two and had learned Japanese basically on the job with no formal instruction. There was a teacher he worked with who he thought was cute, so he kept trying to make nice comments and flirt with her. One day that backfired.
She was wearing a shirt with some kind of cartoon character on it. He, trying to be nice, complemented her on her nice manko. Manga means “cartoon.” Manko means something close to “cunt,” but a bit nastier.
I’ve also heard from the rumor mill that one girl called her Japanese friend because she was having trouble making her curry sauce (karê) thick enough. Her friend gave her several points of advice, including putting on lipstick, getting naked, and touching the sauce “down there” before the poor girl understood that what she’d actually been saying was that she couldn’t make her boyfriend hard (kare wa kataku naranai).
One of my friends used to play practical jokes on people who thought they knew Japanese but really had only a basic understanding. He told them that in his area the regional dialect rendered genki desu ka (lit.: are you healthy? [how are you?]) as benpi desu ka (are you constipated?)
The last blooper I’d like to relate is not from someone I met, I read it somewhere, but the story stuck in my head and it fits this topic perfectly. A young woman had just graduated from college and took a job teaching English in Spain. She was teaching a class of late-teens to early-twenties men who had only a passing interest in learning. During her self-introduction she started trying to translate what she was saying because they didn’t seem to understand her English very well. Everything went fine until she translated “I have a rabbit” as “Tengo un conejo” (I have a pussy). They seemed a lot more interested in the class after that.