Back when the US Pres election was in full bore this was kicked around a bit - I posted this then, and am reposting it now, worded a bit differently:

If I were the guy who wrote the original program for a balloting machine, I'd set up ifchecks within the system so that my political party would always win. Something to the effect of 'democrat votes == dem, republican votes == rep, if dem>rep, rep = rep +1000, then redo the if'

Folks, nobody is going to dispute the integrity of a machine. Folks always assume the machine is telling the truth - if it's programmed to lie, it's very difficult to prove that lie... Even if we ran a paper ballot alongside the corrupt computerized one, it'd be *impossible* to tabulate and compare the physical ballots with the electronic ones. There would always be a discrepancy.