Perhaps the most stunning result would be an electoral college tie, which would require members of the next House to select the winner. It's not likely, but certainly possible.
If Barack Obama carries every state that Democrat John Kerry won in 2004, plus Iowa, New Mexico and Nevada, then he and John McCain each would have 269 electoral votes. A tie also would result if McCain takes New Hampshire from the Democrats' column but loses Iowa, New Mexico and another state that Bush won, Colorado.
Both scenarios are conceivable, though hardly probable. The Constitution directs the newly convened House to settle such ties, with each state delegation having one vote. Wyoming's lone representative could offset California's 53.
All 435 House seats will be up for election, so the next Congress' partisan makeup is unknown. However, House Democrats now control 26 state delegations to the GOP's 21 (with three evenly split), and they will be favored to keep or expand that edge. If so, Obama almost surely would survive a tie-breaking vote.
The only presidential election that resulted in an electoral tie was in 1824, when the House chose John Quincy Adams over Andrew Jackson.
More likely scenarios could involve Election Day confusion, long lines and possible legal challenges at various polling places. Such scenes will have limited national impact unless they occur in a state where the Obama-McCain contest is extremely close and positioned to determine the overall outcome.
That's what happened in Florida in 2000. A host of ballot problems and a whisker-thin margin ignited one of history's most contentious election challenges. The Supreme Court eventually ended it, sealing Bush's electoral college win over Al Gore, who won more popular votes nationwide.
Congress responded to the Florida mess by passing the Help America Vote Act, which devoted about $3 billion to new voting equipment and other measures. Most jurisdictions welcomed the upgrades. But election officials note that millions of Americans will be voting on equipment that is new to them and to precinct workers.
Record turnouts could overwhelm poll workers and voting machines in some precincts. The trend toward early voting in many states, however, should ease the problem somewhat.
Far more Election Day problems stem from human errors than from equipment malfunctions, said Kimball W. Brace, president of Election Data Services Inc., a Washington-based political consulting company.
"Every system is reliable if people know how to work it," he said. However, he said, "the American public has a great way of fouling up their ballots," which can lead to legal challenges and disqualified efforts to vote.
In one commonplace error, people accidentally vote for two candidates for one office. Some systems alert voters to the mistake before they leave the booth, while others do not.
Republican and Democratic activists will look out for unqualified voters and for shenanigans designed to impede legitimate voters in key precincts. A quarrel, equipment snag or legal challenge that halts voting even briefly can cause people in long lines to lose patience and leave.
In past elections, Democrats say GOP operatives have used disinformation and scare tactics to try to suppress voting in heavily Democratic precincts, including predominantly black neighborhoods.
Democratic Gov. Ted Strickland of Ohio said Republicans are trying to frighten newly registered voters in his state by filing numerous lawsuits that question their eligibility. GOP officials say they simply want to avert voter fraud.
In Florida, thousands of new voters may be unable to cast ballots because of discrepancies between their registration forms and government records like driver's licenses. And in August in Florida's Palm Beach County, which was using new voting equipment required by the state, officials lost 3,500 ballots in a close judicial race. They eventually found them, but it took three recounts to declare a winner a month later.
The McCain and Obama campaigns have poured substantial resources into heading off problems and streamlining voting by their supporters.
"We've been sending our scouts out months in advance" to look for possible trouble spots and to consult with election officials throughout the nation, said Jenny Backus, who is working for the Obama campaign on voter protection. The campaign has conducted numerous "know your rights" rallies for potential voters, she said, and it has armies of volunteer lawyers ready to jump into legal disputes.
The Obama campaign will pursue a "selective but ferocious legal strategy," Backus said.
The McCain campaign has similar plans.
"We will have people across the country - volunteers, poll watchers, attorneys - paying attention to issues we've seen in the past on Election Day," spokesman Ben Porritt said. They will focus on "hot spots" where "we have seen examples of fraudulent activities," including efforts to register unqualified voters, he said.
Finally, in the unlikely event that Washington state determines the Obama-McCain contest, the nation could be held in suspense for days. The state's vote-by-mail system allows ballots to be postmarked as late as midnight on Election Day.
Polls show Washington leaning strongly toward Obama.