The Foo Fighters' tour has been replete with unexpected sights: impromptu birthday celebration beer-chugging, Westboro Baptist Church Rickrolling and -- although we’ve become somewhat desensitized to it at this juncture -- Dave Grohl has been sitting on a massive, guitar-wielding throne throughout the entire jaunt.

Nevertheless, Grohl and company have maintained a firm grip on the element of surprise, and earlier this week during their concert in Noblesville, Ind., that surprise came in the form of a guest appearance from Blues Traveler’s John Popper. Popper appeared onstage for a roughly five-minute harmonica jam session, which can be seen below (and Grohl can be seen loving every minute of it):

The Foo Fighters’s tour continues tonight (Aug. 29) in Chicago. They have more North American dates scheduled through mid-October, including an appearance at Austin City Limits. Check out their entire tour itinerary right here.