Based on preliminary reports published by the team of experts that New Hampshire engaged to examine an election discrepancy, it appears that a buildup of dust in the read heads of optical-scan voting machines (possibly over several years of use) can cause paper-fold lines in absentee ballots to be interpreted as votes. In a local contest in one town, preliminary reports suggest this caused four Republican candidates for State Representative to be deprived of about 300 votes each. That didn’t change the outcome of the election–the Republicans won anyway–but it shows that New Hampshire (and other states) may need to maintain the accuracy of their optical-scan voting machines by paying attention to three issues:
Routine risk-limiting audits to detect inaccuracies if/when they occur.
Clean the dust out of optical-scan read heads regularly; pay attention to the calibration of the optical-scan machines.
Make sure that the machines that automatically fold absentee ballots (before mailing them to voters) don’t put the fold lines over vote-target ovals. (Same for election workers who fold ballots by hand.)