when you hold a music disc in hand and click with it on a jukebox many times very quickly, with a signifigent amount of lag, it will play the song with no disc inside the jukebox. nothing stops the song from playing except for leaving the game and joining again. i tried it on a survival server i play on and the same thing happned, but no one else could hear the song, so its a client side thing. this is a VERY small issue, i dont really care if it gets fixed or not just wanted to let you know.