'Troublemaker' singer Olly accidentally told Monica Michael that she would be going home before reading out the results from the public vote.
Dermot, who stepped down from X Factor in March after eight years on the show, tweeted: "Hey @ollyofficial Ahoy my friend. Keep that head up. Happens to us all."
Olly replied: "@radioleary cheers Derm!! Beating myself up about it mate! What a plonker!"
Dermot later added to his followers: "Didn't see the show tonight, but live TV is a tricky old beast at times. After this weekend... perhaps some sense of context should prevail."
Olly had already taken to Twitter about the slip up, apologising for the mistake once again: "I apologise to everyone tonight I made a massive f%#k up at the end. It was so tense!! I'm still learning & hope that never happens again!"
Hey @ollyofficial Ahoy my friend. Keep that head up. Happens to us all. X— Dermot O'Leary (@radioleary) November 15, 2015
Dermot, who is making a return to Saturday night TV as the host of a brand new BBC show The Getaway Car next year, said in September that he hasn't been watching the new X Factor series "religiously" because he now has his "weekends back" after so many years.
Dermot was replaced by Olly and Caroline Flack. The pair had previously presented spin-off show The Xtra Factor together for two years on ITV2, and became a popular and much-loved presenting team.
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