The check-out people in our supermarket tend to respond to "Thank You" with "Thank You", and customers are likely to respond in like manner. It can go on for quite some time as they bat it back between each other. Especially if you are in the queue behind them.

I dislike the kind of grammarian who imagines there is just one way of saying things and the rest are plain wrong, but I love the ones who help us understand what the variations can tell us about who we are and where we come from.