Two professors demonstrated to a Florida county that they could change the votes recorded by Diebold machines without leaving a trace. The demonstration they designed is badass:
To test the machines, Thompson and Hursti conducted a mock election on systems loaded with a rigged memory card. The election consisted of eight ballots asking voters to decide, yes or no, if the Diebold optical-scan machine could be hacked.
Six people voted "no" and two voted "yes." But after scanning the ballots, the total showed one "no" vote and seven "yes" votes.