Always put any bigger jobs in writing, as previously stated, lack of clear communication. If the proposal is SPECIFIC about the work being done, and signed, the cheap homeowner is less likely to try and pull that sort of thing on you. Theres always the cheapskates out there that will take advantage of your laid-back verbal agreements. I got burned real bad on a job this summer, turned into a big argument over what was included in what price, completely my fault, will I let it happen again? absolutely not. Take this as a great lesson.
Look at it this way, your chances of letting this happen on a larger job where more money could be at stake is probably less. Each lesson you learn is hopefully another costly mistake you avoid in the future.
Ive learned the hard way one too many times.