I can't speak to all counties only my own. I've been an election official for 20 or so years and take my position very serious. My county has roughly 80 polling locations for 200,000 registered voters. We have at our disposal 600 voting machines. This machines are from a multitude of manufacturers. These machines are rotated randomly throughout the county for each election. We can use as many as 13 different ballot designs each election, but usually stick to 5 or 6. Ballots are unique to the polling location in which you are able to vote. Bar coding, watermarks, paper thickness and translucency are ways we can validate an individual ballot to a polling location. Once calibrated, the machines will not tabulate a ballot not meant for that location. Mail in ballots fall under the same ballot criteria. As far as I know, only three people know what machines will be a which polling location for each election and they are kept under lock and key until election morning. Early voting uses a different set of machines per location each day and they are randomly moved each night after the days totals are tabulated. I would like to think this is a common practice across the US.
