This sort of happened to me here in Cachoeira Paulista. It’s a small town and people are not really concerned with the environment, so they are not used to the concept of plastic bags being bad. Besides, everybody uses them for trash, so people usually carry a few more than they need when they leave the supermarket.
So this time I went to the supermarket with my reusable bag (as I always do), and when I noticed the guy was packing everything in plastic bags and putting the bags inside my bag.
But the good thing is that it appears that our — mine and my girlfriend’s — weirdness paid off. They now have a line in the supermarket where they don’t offer plastic bags! Instead they offer leftover carton boxes from their products for people to carry their stuff. Of course the main motivation behind this is a law that will prohibit plastic bags in the state some time in the future, but at least we made them conscious of the need for reusable bags.