1. Before our age of GPS maps and precision surveying, some map makers used to purposely put in little artificial defects into their roads or neighborhoods (non functional little things). This was so that someone who couldn't be bothered to do their own survey and copied the map exactly would not know the difference and reveal themselves by the defect (and be sued for copyright, etc). Such as little hooks on ends of roads, or even fake small villages in the middle of nowhere.
In fact, I recall one story where a village actually became real after enough companies had copied a certain map issued by someone who had put in one such fake village to defeat copying, and a general store named itself after the place, thinking it must be real.
2. When people plagiarize code without understanding it, leaving in non-functional variables or loops that the original author either didn't delete or put in as a trap.