At some point, there's bound to be a misunderstanding or two with your client. Do not despair! Careful documentation along the way allows for a quick resolution if you mutually review the paper trail (hard copy or electronic).
Sometimes your client may become angry because of something you have or have not done. If the issue is your fault, such as a failure to deliver according to the timeline you both have established and signed off on, then own up to the mistake and figure out how to make amends. Often, if the fault is your own, the remedy will come out of your profits.
If the fault lies in the client's camp, then politely reconstruct the situation with your files and documents and offer solutions for solving the problem. In this case, get your client to agree to pay any additional charges that you will incur as you take the necessary steps to make things right. And get it all in writing.