it was the war of Hogwarts.

Voldemort: "Sing or Umbridge dies."

Hermione: "I don't know this song."

Harry: "Me too.."

Umbridge: "Oh come on, sing it now.."

Ron: "I have no clue too.."

Fred and George: "Us too."

Voldemort: "AVADA KADAVRA!"

Umbridge drop dead, and you smirk, sit on a broken chair, and you sing the song after her death, the song is 'Can't Hold Us' by Ray Dalton and everyone was amused, because YOU knew the song, but you didn't sing because you wanted Umbridge to die, clever.