The Benny Hill Theme

Clara Oswald added a piece of music she called "The Benny Hill Theme" to a video of the Mire being defeated in a Viking village. The Twelfth Doctor threatened to circulate the humiliating footage unless the remaining Mire left Earth. (TV: The Girl Who Died)

Behind the scenes
Like Clara, many people in the UK often refer to the music as "The Benny Hill Theme", due to its use as the theme tune for. In reality, the piece is titled "Yakety Sax" and was first performed by American saxophone player Boots Randolph (the song's co-composer) in 1963. The song was further popularized by American guitarist Chet Atkins as "Yakety Axe"; lyrics were later added to the song when Atkins re-recorded it with Mark Knopfler.

The fact Clara is able to overdub the music over a video implies her phone has an app to do this. Although no such app exists (yet) in the real world, there are websites such as "Benny Hillifier" that will add the music to streaming videos. A popular Internet meme is the concept (invoked by Clara) that "everything is funnier with Yakety Sax", hence the music sometimes being used to accompany otherwise-serious footage of disasters and other destruction.

In the TV show, the theme is typically heard over fast-forwarded scenes with odd antics that play during the closing credits; it is this effect that is invoked in the Doctor Who episode.