The 38-year-old judge almost destroyed schoolgirl magician Issy Simpson’s performance during the second semi-final live show.
During the performance Issy asked the judges to pick a random celebrity, a favourite time of day, some change from their pocket and a number on a scrap of paper.
Alesha was heard reading out the wrong number from a scrap of paper she has taken from the eight-year-old hopeful as part of the trick.
She accidentally read the number 30, 0517 as 30, 0515 which would have undermined the end of the trick because the digits corresponded to today’s date.
Luckily, David Walliams quickly corrected her and saved the trick for the Britain’s Got Talent hopeful.
Afterwards, David, 45, chided Alesha further as he praised Issy and said: “Alesha very nearly ruined for you but reading out the wrong number - not good to give her that kind of responsibility!”
Alesha Dixon almost ruined Issy Simpson's magic trick on Britain's Got Talent
Alesha Dixon makes a blunder on Britain's Got Talent
It’s true, I messed up because I was sitting here, I couldn’t believe my eyes
He continued: “You are a real star and that was an incredible trick. I think people are going to vote for you.”
“It’s true, I messed up because I was sitting here, I couldn’t believe my eyes," she said defending herself.
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However, David made fun of her, saying that she might have had a pre-show tipple - forcing her to tell Issy: “I’ve had nothing to drink!”
Issy Simpson performs in the Britain's Got Talent semi-final Britain's Got Talent: Most memorable moments Mon, May 1, 2017
Britain's Got Talent: The best and worst moments from the ITV talent show.
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David Walliams was not impressed with Alesha Dixon on Britain's Got Talent
He jokingly picked up her drink and took a sip before spitting it out in a comical manner to imply it was not water.
Despite the blunder, Issy along with fellow magician Matt Edwards made it through to the final after receiving the most number of votes from the public.
Yesterday Simon nearly ruined DNA's performance when he was having trouble locating a word in a dictionary.
Britain’s Got Talent continues on ITV tomorrow night at 7.30pm.