The singer messed up after coughing during his rendition of Rihanna’s hit ‘Diamonds’, which resulted in him losing his way in the song.
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A seated Simon - who originally picked Calum as his Golden Buzzer act - was less impressed though, telling him: "Oh Calum. It fell apart. You lost your words, you lost your timing, luckily on the second half of the song you picked it up.
“I do think you're a really good singer, though" he added.
Alesha added that those sort of mistakes happen to any seasoned performer, telling him: "It happens. It's done. You have the most magical voice."
A dejected looking Calum was also given reassurance by hosts Ant and Dec, before apologising to Simon for letting him down.
He’d previously wowed the panel on Friday night’s semi-final with his performance of Jermaine Stewart’s ‘We Don’t Have To Take Our Clothes Off’.