Waterloo - Mr. Burns and Smithers
Waterloo is a song by Swedish band ABBA. It was first performed by the band at the 1974 Eurovision Song Contest, which it won.
It's played in Mr. Burns' tank in "Mother Simpson" and it's revealed that Mr. Smithers had put the song on the cassette tape and had taped over the original track on the tape Ride of the Valkyries by mistake. Due to Mr. Burns mentioning he had waited 25 years to play Ride of the Valkyries before discovering it was taped over by Smithers (the episode at the time airing in 1995), it can be assumed that Smithers recorded the 1974 broadcast.