Mike played the Let It Go piano part at the end of the ballad medley and guitar on In The End.
Don't know if this happened before but Chester sang at the end of Welcome.
and a really strange fact: LP (or management?) didn't allow smoking in the backstage area. some bands said that during their set and you can see it here https://twitter.com/Kraftklub/status/635475523255332864.
I don't know who really was the initiator but what a dumb regulation for an open air festival...