I needed to install a new, outside faucet this weekend and I had my teenage son helping. I told him he had to learn to solder plumbing because you just never know when you'll need that skill.
So as we were working, it became time to cut the existing water line so I marked the location and handed him the saw (I couldn't find my tubing cutter). He said he didn't want to do it. At first I thought he figured out that he'd get wet but I asked him why, none the less. He said that he couldn't take the pressure of disabling all of the water into the house. I thought that was an interesting thought but he had a good point -- never start a plumbing project you can't finish.
That became Rule #1 of Plumbing: Never start a plumbing project you can't finish.
Rule #2: For project of less than one day in scope, add at least 3 hours and two trips to the hardware store to your estimate. For longer projects, add 1 day per estimated day. Wives scream less if you finish early.
Rule #3: When the pipes are cut, the plumber is wet and things aren't going okay, it is okay to lie to people about anything you want if it gets them to leave you alone.
Rule #4: Always give your wife a bill. She'll blow you off, but at least she might see that whatever you did had some value on some planet.
Rule #5: Don't tell anyone when you're done so you can get the first shower.
I'm sure there are more.