If a Florida resident is identified as 'Bill Smith' in one database and 'William Smith' in the other, the voter won't be allowed to cast a ballot, she said. Voters who haven't been validated won't necessarily know until they arrive to cast their ballots.

Some, but not all, ineligible voters have been sent warnings, but those notes don't specify which information on their registration forms is incomplete, Wheatley-Giliotti said.

Ion Sancho, head of elections in Leon County, Fla., said the problem could be compounded in some counties where IT personnel are unfamiliar with the statewide database technology.

The voter database previously used in Leon County was based on the same technology in the new statewide data'base, so IT personnel there can fix glitches, Sancho said. 'Other counties don't know all the ins and outs,' he noted.

Justin Levitt, associate counsel with the democracy program at New York University School of Law's Brennan Center for Justice, predicted that database problems are likely in a number of states, such as Florida, North Carolina and South Dakota, which require that new-voter information exactly match data in other state repositories.