Years ago, when I worked in a movie theater, we had free drink refills and you could refill your popcorn for $.27. (Technically, we charged $.25, and then the $.02 extra was tax. Anyway. . ) A couple times a week, we would refill a cup that had almost certainly been saved from a previous visit, but the worst were the people that would buy the popcorn w/ extra butter, get a refill (again, w/ extra butter), and then save the bag to bring it back the next week. The butter would go rancid, and it would reek. And, if you said anything, the people would swear up and down that they had just purchased the bag.