I once had a customer who set a lawn sprinkler too near his conservatory. When I went to clean the windows the conservatory glass was almost opaque with a layer of limescale.
I knocked on the door and asked the customer to come and look at the conservatory - he said he hadn't noticed that his sprinkler had been watering the conservatory as well as the grass!!
I asked him to run his finger down the limescale so he could see how thick and hard it was (waiting for the comments on that

) and then said if he wanted me to try to return his conservatory to its original condition there would be a very substantial extra charge.
He apologised for being a numpty, I quoted him an enhanced hourly rate and returned a few days later with some Cillit Bang and cleaned up his conservatory.
Result:- a very good (though exhausting) afternoon's earnings, an apologetic and contrite customer who thereafter took care to ensure he paid promptly, never 'not this time'-ed and was polite and grateful for my service.