Fearne Cotton was caught swearing live on air and had to apologise to listeners and red button viewers.
The BBC Radio 1 presenter, 32, was hosting a Christmas edition of the Live Lounge with special guests, comedian Ricky Gervais, TV chef Jamie Oliver and pop star Cheryl Cole.
While the Wizzard song 'I Wish It Could Be Christmas Every Day' was playing, a microphone was left on and online live-feed listeners heard the gaffe.
She told 'Call My Name' singer Cheryl: "No swearing and don't make any w***** gestures because people will see them on the webcam."
The bad language was also witnessed by red button viewers and Fearne later apologised when she realised she'd been caught out.
She told listeners: "We might have heard some bad language on the radio because one of the mics was still faded up in the kitchen."
Coldplay's Chris Martin also performed live for the programme, along with Jonny Buckland.
During the show, BBC Radio 1 breakfast presenter Nick Grimshaw wore a Miss Santa-style red minidress for a round of DJ Scott Mills' "innuendo bingo" which is popular on his show.
For the occasion, Fearne also wore a baggy red Christmas jumper featuring Rudolph wearing a Santa beard.
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