Yeah, its been a while, but I vaguely recall that's how my voting worked: you vote on a touch screen, then review everything, and then your ballot gets printed out. I then bring the paper ballot over to be put in the paper box. I believe I checked my paper ballot, but that was mostly since people were already quite worried about cheating. I'd assume a good number make sure the touch screen is correct, if that, and then don't review the paper ballot. I recall some places had a system were you don't actually even look at the paper ballot.
If you assumed 90% don't check, and you only did it for the 50% voting wrong anyways, and did 10% of those, you probably could get a 9% swing through that, and then you just admit the "error" for the 1% of ballots where its discovered and change it. If you really want to get more through, make the appeal process at the point where the paper ballot is printed obnoxious, or make the only recourse the voiding of the vote: so you don't generate a vote for your guy, but do get rid of a vote for the other guy.
Still, small and complicated compared to looking at who hasn't voted by the end of the day and stuffing fraudulent ballots then.