Dave Grohl was definitely not expecting this fan, who's face was painted like a KISS member, to absolutely kill it on guitar. He knew the song inside and out and his stage presence was unreal. Dave was so blown away with his talent that he was actually distracted by him and forgot the words to the song halfway through!
He even had to clear the air after the fact and publicly state that the fan's performance was not planned, after many fans claimed that it was a planned stunt.
I mean come on, how could you not think a fan whose face is painted like KISS that randomly gets called on stage and plays like a seasoned rock star, was not planned?! See for yourself!