"It seems to me if you were trying to commit fraud, you wouldn't show [the ballot] to the voter," he said.

One of the possible causes of vote flipping may be voters who place their hands on the side of a machine as they vote, perhaps accidentally touching it with their thumb and erroneously making a selection, he said. In other cases, some e-voting machines use a thumb-operated wheel to advance the electronic screens and when it is turned, it highlights a candidate in the next race on the ballot -- possibly giving a voter the impression that an erroneous choice has been made for them, he said.

The way to figure it out, he said, is to bring together a panel of experts to investigate the issue, confirm it, find ways to fix it and then get any fixes out to voting officials, Dill said. Until then, election officials and watchdog groups across the country will continue to hear reports of vote flipping, he said.

"We know it's going to be a major deal," Dill said. If a voter notices the problem on an e-voting machine's review screen, they can try to go back and fix it, he said. Sometimes it takes multiple attempts to correct, according to reports. But if they don't notice it or try to go backward to fix it, their votes are improperly cast, he said.

"This problem, I think, is a national disgrace," Dill said. "There needs to be a serious independent investigation of this problem ... across the country."