In practice, cops in a position to arrest people on their beat are generally arresting a series of people for the same thing over and over again, and eventually

In practice, cops in a position to arrest people on their beat are generally arresting a series of people for the same thing over and over again, and eventually learn the details of what will satisfy a charge. Mileage will vary based on geography and beat though. And... I'd admit we saw some charging documents we immediately dismissed from new officers, roughly like 'this guy seemed shady and was near a crime.' We'd lay into them for that though, they'd eventually seem to figure it out.