What I expected to happen was...:
Normally the music track will come on as a one track playing in the game.
What actually happened was...:
A music track began playing but then a couple of seconds later a different one began also on top of the one that was already playing so the two tracks were overlayed which was odd.
Steps to Reproduce:
1. go on any gamemode and play normally
2. wait untill the music starts to play, they might overlap they might not, it does happen quite often though.
NOTICE: this bug was noticed when playing on the 1.7.2 vanilla server so it might be just servers.