all it would take is one person saying their printed ballot does not match their specific selection, and the whole thing would become chaos.
Only if the scantron shows that each position on the ballot was counted and the voter is not allowed to leave until the person monitoring the scan confirms with the voter their ballot was scanned would this give confidence. Any issues with the scan, and the voter is allowed to correct the issue. There should never be an issue of reading the ballot by the scanner as an acceptable outcome.
of course, all of this is assuming in person voting only
It might slow things down a little bit, but making sure that the machine can detect a vote for each race/question (even if it's just "Abstain") would make sure people didn't forget to fill out something and help prevent the fill-in-the-bubble equivalent of hanging chads.
Manually counting votes is so error prone that I'd have less confidence in it than a scantron type of ballot. At this point, I'm more in favor of giving each voter a ball/bead/chip to drop into a bucket for each position on the ballot. After checking in, you go to each position to receive your one token. If you don't visit a position, you do not get a token to pass to someone else. Tallying the votes could be as quick as weighing the bucket as the weight of the bucket/token will be known. Each election can change size/weight/color of tokens to be unique. If the weights total an irrational weight, it would be deemed suspect and a hand sort of the tokens can be done to find the odd token.
Balls/tokens aren't a bad idea either though, but it sounds like people pocketing a ball/token would force a manual count even if they kept them since the total weight of all buckets combined would be off. I'd also worry about people bringing in heavier or lighter balls/tokens but the bigger risk would be poll workers handing out heavier or lighter balls/tokens to specific people (or types of people) because it'd be easier to make sure the weights would add up in the end.
Maybe we could force everyone to vote at every position (which should have an abstain option) then have the machine check the weight of every ball/token as it was inserted, and verify that one but only one was inserted, before it fell into the selected bucket?
I like the idea of placing the token into a verifier to validate authenticity before dropping into the bucket. Similar to a coin sorter where invalid tokens get rejected to a separate bin with a light and siren to ID the person trying to cheat. These could get expensive as you'd need one per candidate per position on the ballot.