"Austin City Limits always packs a few surprises, but I don’t think anyone was expecting this: A guitar-slinging Rooney Mara joined the Black Lips onstage and rocked alongside them for one song while Ryan Gosling watched from stage left. Oh, and reclusive director Terrence Malick was on hand filming the whole thing."
There is even some video captured of the event!! Fuse have links to YouTube clips that someone from SlashFilm shot on their cell. Click on the link below to see the clips!!
Source and Source
Thanks to @jillianlowee for the great picture