A building contractor was being paid by the week for a job that was likely to stretch over several months. He approached the owner of the property and held up the check he'd been given. "This is two hundred dollars less than we agreed on," he said.
"I know," the owner said, "but last week I overpaid you two hundred dollars, and you never complained."
The contractor said, "Well, I don't mind an occasional mistake. But when it gets to be a habit, I feel I have to call it to your attention."