A rally outside the Montclair, N.J., town hall on July 1. Protesters hung 1,101 absentee ballots to represent the number of votes that weren’t counted in a mayoral election that was decided by just 195 votes.Mail-in voting, which tens of millions of Americans are expected to use this November, is fraught with potential problems. Hundreds of thousands of ballots go uncounted each year because people make mistakes, such as forgetting to sign the form or sending it in too late.