Many years ago I taught a very large lecture course and to facilitate grading 400 exams I asked the students to write the number of the essay they chose to do on the front of their blue books. I wrote several times on the exam in bold type, Write the number of the essay you choose on the cover of your blue book. After a few experiences I also formed separate piles in the classroom when the students handed in their exams with a sign indicating the number of the essay. This pile is Essay 1. This pile is Essay 2. Etc.
If you've taught – from preschool to grad school – you know that no matter how many times or how clearly you give instructions, some students won't follow them. You can get to the point that most do if you're persistent, but for whatever reason some of them either won't read, won't listen, or won't process them. Of course in the example above it's not a big deal. It wasn't like they failed the exam if they put theirs in the wrong pile, or didn't mark the cover of their book. In that case the only consequence was me and the teaching assistants wondering why they can't follow simple directions to make our job a tiny bit easier.
One of the issues – not problems, but an issue to be aware of – with mail-in ballots is that the instructions create opportunities to reject ballots. To use the example of the recent Kentucky primary, ballots were rejected for failure to sign, failure to sign in the correct place, failure to enclose the ballot in an inner envelope before putting it in the outer envelope…on and on. Petty stuff, but stuff that is going to get your ballot tossed if you don't read and follow directions correctly.
My point is not to stick up for or lambaste Tyranny via Petty Bureaucracy. Rules and procedures, in some form, have to exist during an election. At the same time, I – we all – know that some people will end up doing the procedure incorrectly even if objectively we think the instructions are clear and simple. Burmila's Law: Each additional step in the instructions will remove more ballots from the final tally. Because whether the instruction says "Sign Here" or "Slither up a greased pole and battle the Rancor to submit your vote" someone will forget to do it, or do it wrong.
In an election in which a lot of ballots are going to be cast by mail and it is patently obvious that Trump will use every possible mechanism to try to question the legitimacy of the ballots cast, I worry about the potential for these minor, insignificant instructions will toss otherwise valid votes. No voting system is perfect and the rejection rate on mail-in ballots has been low everywhere it has been tried. "Low" and "zero" aren't identical though. True, votes can get rejected when cast in-person too (by filling in too many votes for a single office, etc). From mail to paper to voting machines, none of it will ever be 100% perfect.
This election is likely to be closer than some of the very optimistic poll results from June-July may have suggested ("Biden +15!!") and every vote is going to matter. It's too late to substantially alter any of the procedures in place for requesting and then casting votes by mail; at this point we can only anticipate how it could matter. Some states, for example, still require voters to submit a paper form by mail to request an absentee/mail-in ballot. How many voters do you lose with each additional step? I'm not sure it can be measured, but it's above zero. Some people will forget. Some will not have a printer. Some will do it but not correctly or in time. Some will misread or not read the fine print and give a reason for their ballot to be rejected.
And this is going to get incredibly ugly, with a large number of mailed-in ballots, if in the end one candidate wins by a very narrow margin and the rejected ballots could – potentially – have made a difference. People will start howling to count them anyway, and states will flat-out refuse. The perfect recipe for an election outcome that isn't broadly perceived as legitimate.