Dead people voting by mail.
I suppose you could have dead people voting by mail. According to the website Alan linked to, 9 states (5 of which were already doing so, and 4 of which did so because of COVID) sent out ballots to registered voters without first receiving a ballot request form. So someone received the dead person's ballot, and was able to successfully forge the dead person's signature, and vote in the dead person's name. For the other 41 states, they would have had to successfully forge the dead person's signature twice, once on the ballot request form, and once on the ballot. It could happen. So who does that favor? The Republicans or the Democrats? The other interesting thing is dead people were voting long before the advent of mail-in ballots.
I think Trump may have alleged that dead people voted by mail-in ballots, but if he did, he wasn't able to provide any evidence that that occurred. By my last count, he is 2-38, and the two cases he prevailed on didn't have anything to do with dead people voting.
The 5 states that sent out ballots to registered voters without first receiving a ballot request form even before COVID have said voter fraud hasn't been a problem. But maybe this year was different because it involved Trump.
Of the 9 states that sent out ballots to registered voters without first receiving a ballot request form, only Nevada was a close race. If I am not mistaken, Trump filed at least one lawsuit in Nevada claiming, among other things, dead people voted, but was unable to prove it.