A row has broken out in the TV show's Music Round about the earliest recorded anti-incest song to get into the pop charts.
Contestants were asked to name the singer and the title of the song that best illustrated a strong moral message about illicit sexual relations.
One finger-on-the-buzzer student said it had to be Sister Sledge's 1979 hit 'We Are Family' but the answer proved wrong.
"Sorry, chaps, wrong answer" question-master Jeremy Paxman replied before stunning the studio audience.
"It's Elton John's 1974 single Don't Let The Son Go Down On Me."