My mind was blown when I heard the 2013 remaster of The Eagle's "Taking It Easy".
Original: https://www.youtube.com/watch?v=s0ZZHNRHA2g&t=1m36s
Remaster: https://www.youtube.com/watch?v=4v8KEbQA8kw&t=1m36s
I never knew there was a backing bluegrass guitar on the track!