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 e-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 although election observers say that more people will vote on paper ballots using optical scan machines than they 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 e-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 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 Avi Rubin, a computer science professor at Johns Hopkins University, 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 e-voting machines, a bug could be far more disruptive.
Of the 24 states that use e-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.
Touch-screen calibration errors
The most widely reported problem in early voting so far this year is what's known as a touch-screen calibration error. This is an issue that has popped up in earlier elections as well.
You may already be familiar with this problem if you've used an automated teller machine or a personal digital assistant. Voting machine touch screens have to be calibrated so that the computer knows what part of the screen is supposed to represent each voting choice. The problem is that the angle at which you view the touch screen affects where you touch it, so a 6-foot-5-inch voter may touch a completely different part of the screen from someone who is 5 feet tall. Another source of error: when people touch the screens with different parts of their fingers such as the fingernail. When the voting machine isn't calibrated properly for the user, you can get vote-flipping: where the voter thinks he's selecting one choice, but another one shows up on the screen.
This year, calibration errors have already been reported in early voting in Texas, West Virginia, Colorado and Tennessee. And they've made it into a widely watched Simpsons clip where Homer Simpson tries unsuccessfully to vote for Barack Obama.
Training problems
If the machines don't malfunction on their own, unskilled poll workers might just help them along. In fact, election officials in Florida have reportedly said that poll-worker error is partly to blame for the ballot scanning problems already reported in Florida.
One of the side effects of having so many new voting systems in play this year is that poll workers may not know how to use them properly. This can lead to unexpected consequences that could affect the outcome of an election. In a recent study of Sequoia's AVC Advantage voting machine, university researchers found that because of a design flaw in the system, a poll worker could press the wrong button and cause an incorrect primary ballot to show up for the voter, leading to Election Day mistakes and, possibly, voter disenfranchisement.
Election equipment is certified by the federal government, but there is no user-interface testing as part of this certification process. So often, design bugs don't really show up until Election Day.
Human error
Poll workers can make mistakes, but at least they get some training before Nov. 4. Not so with your average voter.
Just ask Oprah Winfrey. Her presidential vote initially didn't get recorded when she voted using a touch-screen machine in Chicago Thursday. She talked about the problem on her show the next day, saying: "It was my first time doing electronic ... I didn't obviously mark the X strong enough or I held down too long … when I went back to check it, it had not recorded my presidential vote."
After a brief meltdown at her polling station, Winfrey caught the problem in time to vote for her candidate.
But University of California computer science professor David Wagner said that bad design choices could be rooted out if the federal government included user-interface testing as part of the certification process.
Proposed next-generation voting standards would require this type of testing, but it's not clear whether these standards will be adopted, Wagner said.
Voter registration database problems
Database issues may play a more important role in this year's election too, as many states have recently met HAVA guidelines that forced states to set up a centralized computer database of registered voters by Jan. 1, 2006.
Whether these databases will work properly or make voting difficult remains to be seen, but there could be a lot of voters who show up on Tuesday only to find that they are not registered to vote.
States are using the HAVA requirement to clean up their voter lists and are knocking the names of ineligible voters off the rolls, but some states -- Florida, for example -- have tougher requirements than others, Wagner said.
The Berkeley professor said he'll be watching these voter registration databases closely on Tuesday.
"I don't know what to expect," he said. "Everything could go smoothly, or we could have a substantial fraction of voters who show up on Election Day, think they're registered and are told that there is some problem with their registration."
Malicious attack
Although this is certainly the most frightening potential problem, it's frequently downplayed by e-voting experts, who say that mechanical glitches are much more likely to happen.
Still, it is possible to hack the election.
States like California and Ohio have sponsored tough audits of their e-voting technology, and they universally discovered that a motivated attacker could change election results on virtually every voting machine that was tested.
"It's something that anybody who has technical skill could do," said Ed Felten, who has looked closely at security problems with Diebold and Sequoia voting machines.
Because voting systems vary from county to county, it would be hard to rig a presidential election in any kind of widespread way, but concerns will not go away until more states make it impossible for election results to be changed with a couple of keystrokes.