Of course your contractor seemed civilized and competent when you first met with him. That's why you hired him. But sometimes a person can seem like Jekyll initially and then promptly start acting more like Hyde.
It's best you spot this Hyde-like behavior early on, when you haven't: a) shelled out a ton of money for incomplete or shoddy work; b) gotten far enough down the line with the contractor to make it difficult to get out; or, c) driven yourself nearly batty with frustration and/or rage.
A bad contractor can do anything from buy shoddy materials and pocket your extra dough to use drugs and alcohol on the job, endangering himself and others. How do you know when it's time to replace him with a better model?