Years ago, I moved within the County, to a new voting precinct. I notified the Court House of the move. On Election Day, I went to the new polling place. I wasn’t on the roll. I voted a Provisional Ballot. I later learned that Provisional Ballots are almost never counted. They throw them away, claiming that it wouldn’t change the outcome, so why bother.
That attitude exists for mailed in absentee ballots as well. a mailed ballot can end up anywhere, lost, misdirected, thrown in the trash, destroyed. Only one of the possibilities results in a ballot received and counted.
I think in 2008 there were thousands of absentee ballots from our military personnel that were not counted.
In 2012, I was watching the late returns and PA and OH were a couple of the last States to report some Counties. Those Counties both showed more votes for Obama and totals more than registered voters.