I had my students prepare a short presentation on a holiday they celebrate with their family, and one chose the Chinese New Year. At the end, I gave the others a chance to ask questions, and one boy raised his hand.
"But..." he hesitated, thinking. "How do you manipulate the baguette?"
I was entirely baffled; I was imagining breaking bread, perhaps in some unique, creative way somehow connected to the Chinese New Year...but then I remembered. Baguettes is the French word for (among other things) chopsticks.
"Chopsticks!" I exclaimed. "How do you use chopsticks!"
Problem solved.
Another classic occurred today when I was asking the class to list characters in a royal court in medieval times. One girl suggested "valet". I replied that this was the middle ages, and there were no cars yet, but perhaps we could list "carriage driver." Little did I know, "valet" also means "knave" in French...
These are actually wonderful illustrations of how much you can learn about a foreign language simply by examining mistakes its speakers make in your own native language!