they add in extra parts arbitrarily that weren't on the studio recording and add absolutely nothing to the song, usually when the instrument wasn't in the original. Because the person playing said instrument has nothing to do while playing the song, they write something which actually detracts from the song.

Eg MGMT.
http://www.youtube.com/watch?v=RMISSK-zXZc
(Especially at 3.02)