Dave fell off the stage and broke his leg during a show. While they were treating him backstage, Taylor led the band in a short set of covers, then Dave came back out. He said his leg might be broken, but he can still play guitar and scream, and they finished the show. Unfortunately, Foo Fighters did end up cancelling the next several weeks' worth of shows so he can recover properly.
I hadn't heard about or seen that throne, though. Since this was apparently that same night, they must have put it together on the spot.