Response to post in lb:
Everyone who votes a system with a printed "receipt" needs to diligently cross check. This type of error would be "helpful" in a recount against the machine. Those paper receipts are where they will go to get the "real" data in a recount. Don't just rig the vote, rig the recount.
Adding another check:
If you don't want to vote on a machine and demand a paper ballot, ask if they get counted. Last time I did that, many years ago, it was deemed provisional- and in that state, would not be counted unless vote was in recount territory. Which means, cheat big. Recounts, depending on state, may mean only a half of a percent or some lame difference. How many uncounted provisional ballots would have mandated a recount if counted?