A friend services and repairs garden machinery that manicures the Versailles like gardens of Surrey. Some of his customers, rich individuals, rich individuals staff and rich individuals contractors decide to have a go themselves when they are better off with the day job. Most contractors know better than to overreach but some do not.
This provides extra income for my friend as he removes broken off Chinese stud extractors and cures broken brazing and welding jobs carried out by a "mate we know". Years ago when he was young he could not resist a dig but now he an old git like me he keeps a dignified silence and prices the wreckage into the work done.
For other peoples cockups there is gold to be won.