When a joined a new job, in the very first week, I found a few of the colleagues who had started with me bragging about how they found loopholes to exploit in the timekeeping system.
The human mind is drawn to exploiting deviations. Let’s say that a city’s traffic has a compliance rate of 99%. In the absence of penalties, we are drawn to imitate the 1% who offend rather than the 99% who comply. With more offenders, the system can quickly devolve into chaos and turn the city’s roads into death traps.
The same is true of individual systems. It is easier for an environmentally friendly meat lover to turn 100% vegetarian than to be a 90% vegetarian.
Systems need to be ‘watertight’ because deviations tend to multiply. The smallest hole in the hull can quickly expand to sink an entire ship.