All interesting points of view.......and that is why God made attorney's. If something goes wrong, expect to hire a real real good one and expect to lose. The business of excavation/heavy equipment is not a favorite in the public's eye. Ignorance of the law and doing just what the homeowner wants does not exempt you from any liability from the hole that you dug, after all, you are the professional. You are held to professional standards. That hole is yours from start to finish regardless of your agreement stating otherwise. Dig it to code or better, provide a ladder, barricade it off, take plenty of pictures and have your agreement signed........that is the best you can do, but at the end of the day, if someone gets hurt - you will without question be asked to pay.