One just ruined all the goodwill they built up with me over the past year (which got them countless free hours, crunch time at short notice, etc.) by stiffing my company of about a grand - literally a few hours' work - on some badly structured justification, the real reason being he felt his colleague was not negotiating hard enough with me and he "showed him".
Now I'm going to stick to the contract hours and probably prioritise other work over theirs; I also changed the payment terms to be upfront to avoid stiffing, and if they want extra hours, guess what, their sticker price went up from their discounted price for longstanding cooperative clients to "market rate". I guess the thousand bucks is visible and the value they lost isn't, so he probably thinks he was very successful.
It bewilders me that someone can be in business for years and not understand that cooperation is a better strategy, but there are a lot out there. I've found the short term cost of cooperating with defectors worth it in the long run, though.