I was in the south of France for a conference, and tried my best to make use of my (very rusty) French. I thought I was doing okay until one day at lunch, when the waiter returned to ask me if I wanted anything more after my meal. I meant to say, no thanks, I’m finished… so I said, “Merci non, je suis fini,” which is a literal translation. In French however, you’re supposed to say “I have finished,” “j’ai fini.” The waiter politely corrected me, and I thanked him for doing so. He then said in perfect English, “Don’t worry - my best friend is Australian, and he does the same thing all the time.” I felt pretty silly then… Later, I was relating this story to a friend of mine who had spent several years in France. He laughed rather more than I thought was really warranted, and I asked him what was so funny. He then told me that “je suis fini” can indeed mean “I’m finished” or “I’m through,” but more along the lines of “I’m dead, I’m done for.” Great thing to say after a nice meal, no? 
Another friend of mine told me of a time in high school when he wanted to impress a girl that he had a crush on. He’s a native Hindi speaker, she was Bengali, so he thought it would be cool to say something to her in her native tongue. His only other Bengali-speaking classmate, another guy, suggested a phrase that was guaranteed to get her attention. So my friend gathered all his courage, walked up to the girl with a card and a bunch of flowers in hand, and proceeded to say… “I have an enormous dick.” The girl apparently almost passed out in shock at first, but then realized my friend had been set up and roundly abused her fellow classmate for it.
Regional differences in the same language are fun too. One time when I was in Australia, the local park rangers were conducting a round-up of some feral goats that were damaging the landscape. Given the terrain, a number of people with horses and trained dogs were assisting the helicopter with the round-up. I asked one ranger how the dog he had been training performed that day. He replied, “She did well, but she’s a bit knocked up.” I must have been staring like a fool, because he went on to elaborate that the dog was basically worn out. Much amusement ensued when I explained that “knocked up” here in NYC was slang for pregnant, and for a moment I was trying to figure out how a day’s work could make the dog “a bit” pregnant…
Soon after I first arrived in Australia, I was having lunch with my PhD advisor (pasties, yum!) when I needed to wipe off my fingers. I started to ask my advisor if he’d seen any napkins around, and he told me to ask instead for serviettes. There came a moment of confusion as my very proper British advisor tried to make me understand that “napkin” was usually used for “sanitary pad” in Australia, without making any direct reference to feminine hygiene products at all. Priceless! 