a California county uncovered a problem with its voting system software only because the county’s registrar of voters had decided to implement an innovative public auditing program. […]
Humboldt County uses paper ballots that are scanned by an optical-scan system made by Premier Election Solutions (formerly Diebold Election Systems). Under the Transparency Project, after the ballots are officially scanned and tabulated by the Premier system, they’re scanned a second time by a separate commercial scanner, not made by any voting machine company, so that the ballot images can then be posted on the internet for the public to examine and conduct independent recounts.
They use open source software and have documented their project for others who want to replicate it.