When a user goes to the 'Unheard' playlist (from the sidebar) and plays a track, it immediately disappears from that track list! I think this is a little confusing because the track that the user just double-clicked is playing but it is not visible in the track list anymore. 

While I understand that once the track starts playing, it does not belong in the 'Unheard' playlist and that is why it gets removed, perhaps a better way of achieving a similar result is to make the track name grey colored and only remove it from the playlist completely when the user switches to another playlist.