Florida has switched to its third ballot system in the past three election cycles, and glitches associated with the transition have caused confusion at early voting sites, election officials said. The state went back to using scanned paper ballots this year after touch-screen machines in Sarasota County failed to record any choice for 18,000 voters in a fiercely contested House race in 2006.

Voters in Colorado, Tennessee, Texas and West Virginia have reported using touch-screen machines that at least initially registered their choice for the wrong candidate or party.

“I pushed the Democrat ticket, and it jumped to the Republican ticket for president of the United States,” said Calvin Thomas, 81, an Obama supporter who tried to vote early in Ripley, W.Va. “I’m a registered Republican, and I’ve voted in every presidential election since 1948. I don’t like seeing my vote do something I didn’t tell it to do. I take that real serious.”

Mr. Thomas’s daughter, Micki Clendenin, said the same thing had happened to her.

In both cases, poll workers at the site had them touch the screen a few more times, and the voting machine changed their ballot to their candidate choice. State and local officials said these were isolated cases and that poll workers had fixed the problems.

“It was corrected,” Ms. Clendenin said, “but it still made me wonder.”

It was not supposed to be this way.

After the debacle of 2000, Congress passed a federal law, the Help America Vote Act, to avoid similar mishaps. It included money for new machines to modernize the voting process. But in many ways, things have become even messier. The first machines bought with the federal money were largely touch-screens and brought new problems, decreasing public confidence in the process and doubling the number of election-related lawsuits since 2000.

In the past two years, the pendulum has swung away from electronic machines, but the change has come during one of the most dramatic presidential elections in modern history.