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.

-- I know. But last week I overpaid you two hundred dollars, and you never complained.

-- 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.