The Thai show is an unofficial version of the UK show
Lertrak Panchanawaporn correctly answered all 16 questions on the quiz, based on the popular UK show, reaching the one million baht prize (£16,145).
But embarrassed producers quickly realised the answers had shown up on her screen instead of the quiz show host's monitor.
The 44-year-old contestant agreed to replay the game, accepting there had been a technical problem.
She eventually answered six correct questions, winning 25,000 baht (£403).
'Glitches'
"The show organisers told me technical glitches in their computer system gave me the right answers and they could not award me the jackpot," Ms Panchanawaporn told a radio show.
"They told me to play it again. I didn't mind, since I didn't want them to get in trouble,"
During the first game Lertlak used her three lifelines - chances to get help from the studio audience or friends watching at home.
But she noticed during the fifth question that the answer she thought was right had been highlighted in green.
Unofficial
She picked it and continued choosing the highlighted answers for the remaining questions until she had answered all 16.
The problem was blamed on technicians plugging the wrong cable into the monitor used by contestants.
Who Wants to be a Millionaire? has become a hit show around the world as international TV stations scrambled to sign up the show, created by UK outfit Celador.
A spokesman for the company pointed out the Thai version was an unofficial production and not licensed by Celador.
"The computer systems developed by Celador and used across the world within licensed Who Wants To Be A Millionare? productions use sophisticated technology that safeguards against any mistake of this kind," he told BBC News Online.