I had a guest the other day come up and order a large mocha. I grabbed the cup and he said "Large is the small right?" I said, "Large is the large" and then I attempted to explain the sizes.
I also had a guest order "the cheapest size" HCMT. When I grabbed a tall, he argued with me; he said the smallest size was the most expensive. He understood the sizes and was not talking about the cost per ounce or anything like that... he actually thought grande was the cheapest, venti was in the middle, and tall was the most expensive. I think some barista told him the prices but they hit the wrong button when looking at the tall. He was pretty adamant about it.