Asking people to pass you the jam in France will get you raucous laughter. Writing to a host family in Spain can get you uninvited to the house. Why? Due to "false friends" — sneaky words that don't mean what we think they mean.
A Spanish teacher I once studied under told me about her most mortifying language moment. When she was a teen she got accepted to a program that helped American students stay with Spanish host families. Writing to each member of the family, she told a couple's teenage son that she was "ecxitada" to meet him. Having been informed of what the word meant, she wrote again, announcing that she was, "embarazada," and was soon even more embarrassed. "Excitada," does not mean "excited," it means "aroused." "Embarazada" does not mean "embarrassed," it means "pregnant." She had made a couple of "false friends," which is the term linguists use for words that sound similar in similar languages, but have different meanings.
Students often blunder into this dangerous company because related languages have so many words in common. After learning that "frustrated," is "frustrado," and "impatient" is "impaciente," students can hardly be blamed for thinking that "embarrassed" crosses over without a hitch.
We often fail to consider the many connotations of a word, and how different cultures might lean towards other connotations when naming their products. After eating fruit "preserves" much of their lives, American students might not realize that while everyone would like to preserve fruit from rotting, the French might wish to preserve other things as well — which is why preservatif in French means "birth control." English speakers have false friends, too, as anyone who has heard poor French (or Spanish) tourists looking for a "douche" should know. It means "shower," and "tampon" means "stamp." English-speakers are all about the deceptively-named feminine products.
Most false friends are just annoying, but there are a few funny ones. There's the famous "fahrt," which means "journey" in German, but probably gets German tourists some laughs in America. There's a "bad hotel," which in German means a spa, while a "gift" means poison, just in case you're thinking about what to give someone for their birthday. In Czech, the word "host" means the equivalent of the English word "guest." That probably screwed up a few trips.
Do you know of any false friends? Have any gotten you into trouble?