E-voting technology has come a long way since the 2000 U.S. presidential election, when voting equipment problems erased an estimated 1.5 million votes during one of the closest elections in U.S. history.

But progress has zig-zagged. After Congress passed the 2002 Help America Vote Act (HAVA), counties spent billions of dollars upgrading to new electronic voting machines, many of which have now been dumped because they were unusable or, worse still, untrustworthy.

California and Florida, for example, have mostly abandoned e-voting systems in favor of optical scan machines where a paper ballot is scanned into a computer, leaving a paper record of the vote that can be manually recounted in the event of an audit.

That's the gold standard, voting experts say: voting machines that use paper ballots that are routinely audited for errors.

And while election observers say that more people will vote on paper ballots using optical scan machines than did in 2004, there is still room for plenty of e-voting glitches this year in a race that could have the highest turnout in 100 years. Here are a few things that could go wrong with electronic voting on Nov. 4.

Machine malfunction

According to Princeton computer science professor Ed Felten, the most likely election-day e-voting problem is "an engineering error or a bug or a misconfiguration of something that leads to votes either being lost or being put in the wrong column by mistake."

Already Florida has seen some bugs in early voting, where some optical scan machines could not read ballots that were printed on-demand at polling stations. This problem may not happen on Election Day because precincts will be using different ballots that are printed and tested for the machines.

But if similar bugs are widespread in a battleground state, e-voting may be a major issue in the 2008 presidential election, voting experts say.

"We've seen instances in the past where elections were ruined by electronic voting machine errors," Felten said.

Florida's early problems show that the state is experiencing "growing pains" as it moves to the new optical scan systems, but Johns Hopkins University computer science professor Avi Rubin said he would not be surprised if similar problems occur there on Tuesday. "You're coupling an extremely large turnout with many places switching to new equipment," he said.

Florida's optical scan systems use paper ballots that can at least be counted later, but for electronic voting machines, a bug could be far more disruptive.

Of the 24 states that use electronic voting machines, 10 have no state mandate requiring emergency paper ballots to be made available in case of malfunction, according to a 2008 report prepared by voting watchdog groups. These 10 states include battleground states such as Colorado, Nevada and Virginia.