Like everything you do in life sometimes things never go as planned. Sometimes a disagreement occurs between you and your client, and majority of the times its because of a miscommunication.
The simple solution to this is like most things in business is to put yourself in the other persons shoes, have you signed up to a contract such as a mobile phone etc, only to be told something later that makes you think why wasnt I told that before I signed. The answer could be the person selling you the service assumed you understood something and thats an easy trap to fall into once your entrenched into a part of the industry.
When this occurs you are to blame as much as the client is from not asking the correct questions, so breakdown where the problems occurred make sure your contract clearly spells out the solution to the issue and make sure your sales work also highlights this to the client. I provide my clients a checklist I walk through with them, just helping me to make sure I have covered the necessary points. You may loose some customers because of this but your really only loosing customers that are going to be arguments at a later date.