The Help America Vote Act (HAVA) mandated the use of "Provisional Ballots" for voters with registration issues, and it is in place as a "fail safe." After the passage of this Federal Statute, no voter may be denied a ballot.
Voters who are not on the voter roll, or who cannot otherwise be confirmed as lawful in county voters, may not be denied a ballot, but the ballot must be cast provisionally, added to a secrecy envelope with the voter information to be verified later by elecion officials, prior to being cast and counted.
Once a "Regular Ballot" is cast, it cannot be pulled back out, hence the adoption of the "Provisional Ballot" failsafe.
Recap: Lawfully Registered Voters with ID may cast a "Regular Ballot" and unverified/ unverifiable voters may cast a "Provisional Ballot" which may be counted if the voter is deemed eligible.
