Earlier quoted context omitted.
If he's purposely making mistakes to get work in the future then I don't think 6-months in jail is that bad. It's just plain fraud isn't it? Not to mention an expensive inconvenience for all of his customers who have to deal with his shoddy work. On the other hand, it's entirely possible the plumber made an honest mistake.
This sounds similar to planned obsolescence, so the moral is if you design it that way, as a manufacturer, you are ok. Definitely a grey area here. Edit: After some thought, I feel a precedent. My car has parts that don't become obsolete, they flat break requiring never ending service. Surely I can sue for fraud as the auto company has the ability to use another means. (Devils advocate)
For example, Apple noticed that most customers replaced their phones within 2-3 years. A lithium ion battery's lifetime is determined by its charging and discharge characteristics. Its life is extended if it is charged more slowly and neither charged or discharged fully. But this increases charge time and reduced usable battery capacity - both of which are key selling features for smartphones. So Apple optimized the iPhone's battery management to deliver good charging and usable capacity, with the tradeoff that it would degrade significantly beyond 2-3 years of use.
This contractor's activities would have been analogous to Apple introducing code that checks the current timestamp with the timestamp from when the phone was sold and degrading performance when that difference exceeded a threshold.