There's a great netflix special about the drug lab in Massachussets. One lady was just straight up not testing samples at all, and marking them positive for the prosecution. Thousands were jailed this way. The chemist was only caught because her work was 4x as fast as her coworkers, so they investigated what was going on.

Now imagine how many smarter people than her just ran all the tests at normal speed, but faked the result. It'd be damn near impossible for them to get caught. I'd wager for every one caught, there are 10 that don't.