DWIs are the easiest criminal case out there to try. When a cop investigates a robbery or a murder, or really anything else. He knows it is going to trial and he knows his crap needs to be straight.
With a DWI that isn't the case. In fact, 95 out of a hundred DWIs will probably plea. And lots of those that plea were bad arrests where the cop made all kinds of mistakes. And with so few going to trial, some of those mistakes will become ingrained and habitual. So, the cop will get careless and when there is a trial, he is apt to look bad on the stand.
After the trial, the cop will straighten up and do everything by the book again, but it might be a year or more before he has another trial and being human and subject to human nature, he'll slack off again and go back to his old habit.
