All good points here, but the clarification that Snopes and everyone else is missing and not talking about are kind of important:

• “Bullet Ballots” are single votes for one candidate with nothing else filled out. In order to be valid that means…
• A voters information would have to be put on a form and fed into a tabular, and in the case of Georgia and Arizona (I think?) physically reviewed before fed into said machine because…
• The tabulation machines are set to confirm a specific amount of information, and if that information is wrong, it will error. This is a code on the form that can be machine scanned, so that makes sure it fits a specific location, precinct, county, whathaveyou., BECAUSE…
• if someone were to get a grip of these forms and ballot stuff them, then counts would be meaningless.

So really all that needs to be done is to find a large enough sample of size of consistent voters who had a flipped vote, find their forms, ensure nothing is fucky with the forms, then interview to confirm with the voter. Do that for a few thousand people in Wherever, USA and you’ll have your answer.

It’s not hard, it’s just time consuming and costs money. Voters don’t generally have a way to even check their vote was counted for all the candidates they chose in most states, which I think is fucked up, otherwise this might be a bit easier.