Here ya go buddy! They all say it never happens. Already has, and many many many many cases caught last election.
http://kcci.com/article/polk-county-auditor-3-people-accused-of-voter-fraud/7666237?src=app
Did you even read the article you posted?
31 cases in 1 billion votes. I'll take those odds...
Also, mentions that most cases are honest mistakes, not intentional.