WOW! I think the guy I hired read this first and I have photos that would make your skin crawl. Bottom line: he got me for $1900.00. Every single thing he painted had to be completely redone....that's when I discovered he did NOT use the colors I picked, he actually used leftover exterior paint from his mother's house! Because I have pets he said things needed to be sealed first and I did agree to that. What I did NOT agree to was using some kind of foul smelling gray stuff ON MY HARDWOOD FLOORS! THEN he painted them BLACK, telling me that all they were good for was covering over with laminate or carpet. He also dripped and tracked paint all over my ceramic tile floors. PLUS left a wet used paint roller in my garden window and had stuff piled in front so I didn't find it until it had dried. I have no idea how much that is going to cost to repair. Then he left without finishing (thank God) but left the "leftover" paint, uncovered, in the rain. Again, hid it so I didn't immediately find it. Obviously we will be going to court but I doubt if I see a penny from him.
Some contractors work on time and material others on a firm contract. I would never hire the former and am leary of the latter. A contractor may low-ball a bid to get the job planning to make a killing on change orders. If you say good morning to them, they charge you extra for that. If the contract is not very, very specific and extensively fleshed out or if they display their change order schedule prominently on top, show them the door.