When dealing with complex hardware and software, errors happen. In 2008, an Ohio election official discovered errors while uploading the votes from a Diebold subsidiary's machine. When multiple memory cards were uploaded for vote processing, some votes weren't counted. Cards had to be scanned several times to ensure all votes were accounted for. Antivirus software and an error in the machine's programming were both cited as causes of the problem [source: USA Today].
As we mentioned before, improperly calibrated touch screen voting machines can cause voters to register for the wrong candidates. The Florida recounts presented a prime example of dealing with old voting technology. Every questionable vote had to be recounted by hand.
Often voting errors come down to human blunders. For example, when Al Franken won the Minnesota Senate seat in 2008, he did so by 312 votes -- and a state Supreme Court ruling -- after the two candidates fought for eight months. Franken's victory came from close examination of ballots that each campaign challenged for various reasons: weird write-ins, half-marked bubbles, fingerprint smudges and the like [source: Tibbetts and Mullis].
Those sorts of errors don't crop up with electronic voting machines, since software offers a much more guided experience.