This happened once on a bustop with my little brother when i was like 18. they rode up on there little bikes with crisp white shirts in the middle of summer. they started talking and being the awful human beings that we are, we just stared at them blankly when they talked. Prompting one of them to ask, "Do you speak english?" I should explain that though biracial, my little brother tends to look Hispanic/racially ambiguous if you don't know any better. Though I look far more decidedly ethnic, I also favor him a lot. We didn't miss a beat and he answered "Que?" and I answered "Quoi?" They looked confused and rode away without a word.
But there was the summer I was practically stalked by JW's. Good times.
I've never spoken to Mormon missionaries although I've seen them around. Not much patience for JW's either. There was one that I spent torturing for months with Baptist theology, during my "enlightened" phase. It was really funny. She kept saying, "I'm not sure. I'll have to ask one of the elders at the temple." Eventually, I grew bored and ignored her. She's still about, selling her magazines on the street corners.