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.

https://www.npr.org/2020/07/13/889751095/signed-sealed-undelivered-thousands-of-mail-in-ballots-rejected-for-tardiness