UC-Berkeley political scientist Rob Van Houweling, who has written about voting equipment, emails:
I wouldn't expect too much optical scan overvote. If optical scan ballots are centrally counted like punchcard ballots, a lot of overvotes go unnoticed. This was the case in some key counties Florida.
However, most optical scan ballots are precinct counted and the machines that voters feed them into are usually programmed (especially after 2000) to warn voters if they overvoted. Voters then have the opportunity to revote.
There may, however, be a number of overvotes on Minnesota absentee ballots that were (by necessity) scanned centrally.
Also, precinct optical scan machines are usually not programmed to warn of undervotes (especially on down ballot races). Thus, there may be a number of imperfectly filled in arrows on Minnesota ballots that would be discernible to humans but were not discernible to the machines. It is possible, however, that the machines in Minnesota were programmed to warn of undervotes, which reduce this potential trove of votes.
--Jonathan Chait