McDonald's lost that case because they made a decision at the management level to keep their coffee at a temperature that they knew would cause 3rd degree burns in order to minimize the number of free refills customers got. There were internal memos that identified the potential for someone to be seriously injured long before the woman actually was and they chose to continue the practice, that is willful negligence and they deserved to lose.
I don't know the details on the mower case.
If I fill out a T&M and leave it with the valve for a final inspection by the purveyor then I'm covered. If it was good enough for the purveyor, then it's good enough. In order to win a civil case they'd have to prove that the irrigation system contaminated the water and there would be no criminal liability in the hypothetical you described unless they could prove that you faked the report. It's innocent until proven guilty, not the other way around so you don't need to prove you did it, they need to prove you didn't.