The most common cause of mis-vote is operator error. I've watched people use the touchscreen devices; with so many voters both unfamiliar with the mechanics of a touchscreen and unsteady on their feet (the median voter age is about 50), and with the machines made of cheap plastic resting on a table, they grip the machine to stabilize it and themselves. Their gripping thumb rests on the screen and creates phantom signal that makes it hard to localize the touch coordinate.

This is the best argument I'm familiar with against the electronic machines we used to use, and our state switched to paper oval-fill ballots that are then electronically tabulated. But, I haven't seen the statistics yet on how many mis-counts we now have as a result relative to the old system and I'd be curious to see them... Ask yourself how confident you are that 100% of voters know how to successfully fully darken the oval. And since the system is anonymous, nobody looks over their shoulder to verify they understand how to do it before the vote is dropped in the box (though in the event of a close race, a manual recount will be automatically triggered and at that point, at least it'll be human eyes deciding whether the voter circling the oval indicates their intent to vote for that candidate).