I think I told this story here before, but I was once at the Lindt cafe. Those of you who know your Lindt might know that they sell their famous Lindor chocolate balls by the weight. So this one gentleman wanted to purchase a single Lindor ball. When she weighed it, it came up to about 11g, therefore costing $1.20. He insisted that the last time he was there, it had only weighed say 10g and cost $1.10. So he pulled out a different ball and demanded she weigh it. The remarkably patient cashier explained that they would all weigh the same, but he was only convinced after she weighed 3 separate balls.
The clincher was that there was a plate with about eight cookies as free samples. After he verified that they were indeed completely free, he took all eight cookies.