"I was not aware that I had voted no," he said that day in June 2002, asking that the record be changed to reflect that he "intended to vote yes."
The rules allow state lawmakers to clear up a mishap if they suffered from a momentary case of stumbly fingers or a lapse in attention. Correcting the record is common practice in the Illinois Legislature, where lawmakers routinely cast numerous votes in a hurry.
But some lawmakers say the practice also offers a relatively painless way to placate both sides of a difficult issue. Even if a lawmaker admits an error, the actual vote stands and the official record merely shows the senator's "intent."
No one has accused Obama, now a U.S. senator and a leading candidate for the Democratic presidential nomination, of changing votes to play both sides, and an Obama spokesman called that idea "absurd."
But Obama has come under fire from Sen. Hillary Rodham Clinton of New York and former Sen. John Edwards of North Carolina for his frequent use of another oddity of Illinois politics: voting "present" rather than casting up-or-down votes on controversial measures.
"It is very difficult having a straight-up debate with you, because you never take responsibility for any vote, and that has been a pattern," Clinton told him in a debate Monday.
Tommy Vietor, an Obama spokesman, said the mistaken votes were not meaningful. "In Illinois, legislators often have just a few seconds to cast a vote, so after thousands of votes they're bound to make a few mistakes," he said. Referring to Clinton's vote to authorize the war in Iraq and her support for a bankruptcy measure, Vietor added, "The real problem is when Democrats vote like Republicans."
Four of Obama's admitted flubs drew little controversy.
On March 19, 1997, he announced he had fumbled an election-reform vote the day before, on a measure that passed 51 to 6: "I was trying to vote yes on this, and I was recorded as a no," he said. The next day, he acknowledged voting "present" on a key telecommunications vote.
He stood on March 11, 1999, to take back his vote against legislation to end good-behavior credits for certain felons in county jails. "I pressed the wrong button on that," he said.
Obama was the lone dissenter on Feb. 24, 2000, against 57 yeas for a ban on human cloning. "I pressed the wrong button by accident," he said.
But two of Obama's bumbles came on more-sensitive topics. On Nov. 14, 1997, he backed legislation to permit riverboat casinos to operate even when the boats were dockside.
The measure, pushed by the gambling industry and fought by church groups whose support Obama was seeking, passed with two "yeas" to spare -- including Obama's. Moments after its passage he rose to say, "I'd like to be recorded as a no vote," explaining that he had mistakenly voted for it.
Obama would later develop a reputation as a critic of the gambling industry, and he voted against a similar measure two years later. But he was clearly confused about how to handle the issue at the time of his first vote, telling a church group on a 1998 campaign questionnaire that he was "undecided" about whether he backed an expansion of riverboat gambling. And, months earlier, he had voted in favor of a version of the bill.
The senator who led the opposition to the gambling measure, Republican Todd Sieben, said he took Obama at his word that the initial vote was an error. But Sieben also said the thin margin of victory was a sign that perhaps there was more to the vote than met the eye. "He was obviously paying attention to this vote. It was a major, major issue in the state, and it was a long debate," Sieben said. "The inadvertent 'Oops, I missed the switch' -- I'd be kind of skeptical of that."
On June 11, 2002, Obama's vote sparked a confrontation after he joined Republicans to block Democrats trying to override a veto by GOP Gov. George Ryan of a $2-million allotment for the west Chicago child welfare office.
Shortly afterward, Obama chastised Republicans for their "sanctimony" in claiming that only they had the mettle to make tough choices in a tight budget year. And he called for "responsible budgeting."
A fellow Democrat suddenly seethed with anger. "You got a lot of nerve to talk about being responsible," said Sen. Rickey Hendon, accusing Obama of voting to close the child welfare office.
Obama replied right away. "I understand Sen. Hendon's anger. . . . I was not aware that I had voted no on that last -- last piece of legislation," he said.
Obama asked that the record reflect that he meant to vote yes. Then he requested that Hendon "ask me about a vote before he names me on the floor."
Hendon declined to discuss the episode. "I try to block out unpleasant memories," said Hendon, who has endorsed Obama. "If I tried really hard to remember it, I probably could, but I'm not going to try hard because I'm supporting the senator all the way."
Hendon said "it happens" that senators press the wrong button. But he was quick to add: "I've never done it."