The Colorado Division of Gaming says its investigation shows Louise Chavez of Thornton should have won $20.18.
Chavez wagered 40 cents on a machine in the Fortune Valley on March 26 and got the message of the multimillion-dollar payout, even though the machine's top award was posted as about $251,000.
The Division of Gaming says the game software's calculating error was triggered after two bonus games occurred in quick succession while another bonus feature was in play.
"The top jackpot on the slot machine at the time was $251,000, so $42.9 million was way beyond the top price award on that machine," Don Burmania, spokesman for the Colorado Division of Gaming, said.
Burmania says for Chavez to get the maximum payoff, she would have to bet $4. She only bet 40 cents, so her top award would have been $200.
"When you play a slot machine and press the bottom the outcome is pretty much determined immediately by a series of random number generators," Burmania said. "The rest is bells and whistles. In this case it was a lot of bells and whistles that conflicted with each other and gave an erroneous prize."
Burmania says errors like this one are rare.
In 2008, a machine told the gambler she supposedly won $166 million, when in fact she was only due $6.
Division officials have said the casino is not obligated to pay the $42.9 million.
Chavez was unavailable for comment Wednesday.
Her brother told 9NEWS the family retained a lawyer. That attorney did not respond to 9NEWS request for comment.
(KUSA-TV © 2010 Multimedia Holdings Corporation with The Associated Press)