In fact, as Serwer writes:
I spoke with Robert Giles, Director of the New Jersey Division of Elections this morning, who explained that each county in New Jersey has a review board, composed of four individuals, two Republicans and two Democrats, who review each provisional ballot before validating it. In fact, it's fair to say that provisional ballots in general are scrutinized more closely than regular ballots.
"[Provisional ballots] aren't subject to less scrutiny, they're more scrutinized in fact,"explained Gerry Hebert, a voting rights expert with the Campaign Legal Center who formerly worked in the voting rights section of the Justice Department. "Each one of them are generally reviewed by a verification board to make sure that the person who cast the ballot is in fact a registered voter or eligible to cast that ballot." Just generally speaking, provisional ballots are far more likely than regular ballots not to be counted. So relying on provisional ballots to steal an election--really stupid. But it doesn't sound that stupid if you don't know that they aren't (sic) carefully scrutinized.
And it's worth keeping one over-riding point in mind in thinking about all of this: There's far more evidence that valid voters are prevented from casting ballots by overly strict regulations than that invalid voters are casting fraudulent votes.