I sometimes manage construction projects and one of the big ongoing problems is how to deal with a mistake by a contractor or subcontractor.
For example, in one case I had a piece of equipment installed and after having some issues with it, I read the installation manual and the manufacturer recommended 1" diameter pipe be fitted to two of the outlets and on inspecting it I find it has 3/4" pipe fitted. This was not necessarily causing the problem, but nevertheless the installation deviated from the instructions. When the guy came to redo it, he said, "I have installed 3/4" on a lot of these and never had a problem."
In the example above the contractor was nice enough to redo the piping at no charge, but in some cases things can get difficult. If I ask a contractor to redo work that costs thousands of dollars for free because of a mistake, it can cause a lot of grief, especially if the contractor thinks the mistake is not serious or would cause him to suffer financially. In one case, a serious error was made that was not really the contractor's fault because a supplier gave him the wrong stuff and he was facing a $5000 loss, so he asked me "Can you live with it?" Even if a contractor makes an error, nobody likes to clean up their own poop, so it can create bad feelings that can affect future work.
Is there a strategy for dealing with this kind of problem?