I once went to a McDonald's when I was visiting Paris (yeah I know, but it was the easiest option and I was short on time). Since it was in an area with a lot of tourists and my French was pretty rusty, I thought ordering in English would be okay.

Apparently, it's not. I never had such a hard time ordering something. At first, the cashier even spoke in English, but then she suddenly got frustrated, switched to French and pretended not to understand me anymore until I tried speaking in French...