Every project I perform is modular in design, divided into discreet, standalone segments.
At the end of each phase, I will ask you, the client, whether the project is proceeding to your satisfaction.
If the answer is "no," I will endeavor to fix it.
For any phase that doesn't get to a "yes, excellent!" I will not invoice for the work.*
* except for reimburesment of out of pocket expenses agreed to in advance - primarily travel and lodging associated with the project.