Millions of viewers were watching the report from the Sidmouth Folk Festival in Devon when the woman pulled her crop top to one side as she passed by in the background.
But the bungling BBC Spotlight cameraman failed to spot her exposed breasts8 and continued filming the interview in the foreground.
Viewer Dan Lloyd, 23, from Plymouth, said: “It certainly made my day – she was shapely and it was a real treat when she exposed her boob.
“But the film team didn’t seem to notice – or they were enjoying it too!
“They just kept on filming and talking, as she walked past in the background with it all hanging out.”
Others took to Twitter to check whether their eyes had mistaken them.
Sarah said: “Did I just see a breasts in the background of the sidmouth festival accapelo interview?!”
Helen tweeted: “Anyone else see the young lady flash a boob at Sidmouth festival?”
Caroline Cook said: “Oh my god a woman has just flashed her boob in the background of the Sidmouth folk fest on BBC spotlight!!”
Another shared a clip of the broadcast and used a series of hastags describing it as “shocking”.
The live report marked the launch of the 63rd Folk Week at the East Devon seaside town, running until August 11.
Oh my god a woman has just flashed her boob in the background of the Sidmouth folk fest on BBC spotlight!! 😂😂
— Caroline Cook (@CaroOnTheRadio) August 4, 2017
@BBCSpotlight anyone else see the young lady flash a boob at Sidmouth festival?
— Helen Marton (@helenmarton) August 4, 2017
Did I just see a breast in the background of the sidmouth festival accapelo interview?!
— Sarah Cronin (@SarahBurkat) August 4, 2017
Popular with the locals and tourists alike, the event was awarded a prestigious blue plaque earlier this year for its contribution to music.
It involves more than 700 events from headline and intimate live gigs, dance parties, children’s activities, dance displays and local food and drink.