It's called scope creep.
If you are going to give a fixed or relatively fixed price quote, know what is included and have that defined in the text of the agreement (e.g. so many pages, fixed text, three images per page maximum). Also define what is not included (no database connection, no shopping cart, no scrolling images, ...). You have now defined the scope of the work. Some people think scope is just what is included. Good scope definitions also have what is not included so that the client can not misunderstand. An example would be, in-scope, three images, out-of-scope, finding and selecting the three images.
Itemize and number what you will do in the written agreement.
When you consider you have finished the assignment, write and send an "end of assignment" letter. You can also describe how each of the numbered items has been delivered.
If I hire you, and I want to really negotiate you into the ground, I'll get you to agree to a fixed price before all the details are worked out, then I'll include some costly details. On the other hand, if you are doing a "time and materials" contract, then I'll let you know everything, especially the large items up front so I know what it will cost.
Costly lesson, but good lessons aren't cheap. :)