A ballot cannot just dissappear. If you signed in, there has to be a ballot attributed to you. A ballot cannot be counted incorrectly. Either the machine will get fixed and recount all the ballots and/or there will be a hand count. A "spoiled" ballot must be replaced by a good ballot. The normal number of spoiled ballots is tiny, so if someone tried spoil a bunch of ballots and replace them it would be obvious. I think there are also safe gaurds depending on your precinct... And no one is going to risk going to jail over a handful of votes. Voting in person, with ID is relatively secure. Its the unsolicited mail-in ballots you should be worried about.