what is denoted by “spoil”?
Would not read properly. Often marked wrong by voter. Ballot voided and issued new ballot.
It does happen, but not that often. I’ve worked elections for many years. It is only recently that we used electronic counting, prior to a few years ago, be hand counted all from (scan able) paper ballots.
I was election judge again this time in my local precinct, we had a single ballot that did not read properly. The man who cast the ballot had nerve damage issue and it was hard for him to fill in the blocks to be scanned. We gave permission and watched as his wife finished filling in the blocks he had marked (no ballot position changed, she just filled the block he had already marked). Then the ballot was accepted.
Machines are fast, but not very smart. Human interface is another issue.