DNA apologised following the live error after Darren, 29, and Andrew, 43, got their mind-reading numbers mixed up.
Darren asked judge David Walliams: “Without looking at the page you stopped on, do you know the page number?”
David replied that he didn’t know the page number, leaving Darren to write down his guess.
Asking David to tell him what page he had stopped at, he replied: “31.”
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It’s okay because it makes you human
Mortified at his mistake, Darren grinned and nervously laughed: “I got very close. I’m going to change it. I did get it wrong. I got 81. I apologise.”
The pair successfully finished their finale act without another hitch.
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Unimpressed, Simon acknowledged their mistake and announced that he thought their trick was “too complicated”, making him feel “thick”.
Speaking about the mistake, Simon added: “It’s okay because it makes you human.”
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One viewer tweeted: “Awkward when DNA get their trick wrong #BGT #81 #31.”
“Too complicated and confusing and the numbers didn't add up. 31 and 81 are not that close together. #DNA #BGT,” posted another.
A third wrote: “#BGT #DNA was too complex their act. They lost me when he said page 81 instead of 31.”
It comes as 32-year-old pianist Tokio Myers won this year's series.