Probably one in a million chance that it was a mistake. In all the years I worked elections, since 1996, I never knew of one that was a mistake. Mostly it was people who moved and or forgot to register to vote. They thought they could just walk in and vote. Our precinct judge was experienced in this and she always managed to calm them down, get them to fill out a provisional ballot, and assured them that this would register them to vote in the very next election. Most went away happy, very few threw a hissy fit. Never heard of any that were counted. Rules are rules and you must be registered about a month before the election in Texas. But that rule seems to be changing in many states. Rumors were that Beto was bussing in people, to get them to vote, each one had a $100 dollar bill in hand. I have no proof, just many rumors.
I would say a large majority would get tossed since they were not registered to vote.
If they counted 10-20K worth of provisional ballots, first I would be surprised there would be that many and 2nd according to election laws in these states, that would be fraud.
In Arizona: Supposed you request a mail in ballot. You don’t fill it out, but the county knows you were mailed one. The poll workers will see you requested a mail in ballot. So they will hand you a provisional ballot. They are then supposed to check to make sure you didn’t ACTUALLY submit a mail in ballot before counting the provisional ballot.