Sometimes, in single player mode when the environment music is playing in the background and you pause the game by pressing esc it wont stop playing; it still keeps playing. Two pieces of music even three can play at the same time. This different type of music can't be muted by turning music volume down.
The music doesn't even stop when you leave the save; it still plays everywhere including in the title screen and when you enter servers.
Doesn't happen with Jukebox music.
It happens in creative mode, in servers and in survival mode.
The condition that makes it happen is usually if you open a world, a music song happens, you close the world, open a new save or server and then the same music piece plays and it doesn't stop if you leave the save.