Probably the biggest problem I ran into while building ponds was water leakage. What pond builders, whether professional or do it yourselfers, fail to realize is that ALL ponds settle. How much depends on where in the world you live. Here in New Orleans I have seen ponds settle as much as 15" over several years because we live in a swamp. When you are building your pond, elevate the sides as much as possible so the inevitable settling does not cause major problems any time soon.
One of the earliest problems with pond settling is water leaking off the back or sides of the waterfall. If your water level drops quickly and for no known reason, turn off your waterfall and fill up your pond. If the water level has not dropped the next morning, you know water is leaking from your waterfall. Usually it is a simple matter to find the miscreant rock and straighten it out. Even if you do have to call someone to move rocks around, you have saved yourself some money by troubleshooting the first part of the problem.