I've spent nearly every holiday in France, and there I always make lots of mistakes when talking, because I have to speak fast on not to think in while.
Once I asked for a "crêpe salade" (salad crêpe) instead of a "crêpe salée" (salt crêpe; there are either salt and sweet) because the Italian equivalent of "salée" is "salato", that sounds much more similar to "salade". Luckily the waiter did understand me; then I explained him and we laughed.
A terrible misunderstanding was caused by my inperfect French pronunciation. I was in a marekt in Alsace, a French region next to Germany where people speak French and German as well. Most market stall holders are immigrants, you know; so there I couldn't understand much what they said, but I had to since my mother (who doesn't speak French) wanted to try a sweater on. The stall holder was saying something in French too rapidly to let my understand, so I asked him «Pouvez-vous parler plus lentement, s'il vous plait?» («Could you speak slowlier, please?»); unfortunately he misunderstood and thought I asked «Pouvez-vous parler plus l'Allemand, s'il vous plait?» («Could you speak German, please?») and started speaking German, a language I do not understand at all, as fast as before...!