Here's one that's kind of --ahem--different. When I was working in China, I blatantly used every opportunity I could to show off my (limited) Mandarin language skills. So, one day in a group of about a dozen Americans, I began ordering breakfast. When I ordered my pineapple juice, the waiter looked stunned and kept shaking his head, "NO!"
I kept insisting (knowing full well that they did have pineapple juice) but he kept shaking his head. This went on for a couple minutes, he becoming ever more agitated. He finally he took his hands and made a gesture like he was following the outline of two hills, or maybe, the humps on a camel. That's when it dawned on me. The words for pineapple and camel are strangely alike in Chinese (phonetically, it would be like low-ah tow-ah vs. bo-ah lo-ah).
I had attempted to order pineapple JUICE, but I was ordering camel PEE!!!
Between my red face and about a half dozen other waiters cracking up as the story was told over and over and over while we sat there all morning, even though the Americans didn't understand fully, there was no missing that I had made a HUGE mistake.