At a music camp someone sang the French song "A La Claire Fountaine" (pardon my spelling). When I woke up he was still singing it.

And there's also the children's song The Song That Never Ends:

This is the song that never ends
It just goes on and on, my friends.
Somebody started singing it, not knowing what it was,
And we'll go right on singing it forever just because
This is the song that never ends
It just goes on and on, my friends...