In one scene, Marshall mentions that Deep Purple is looking for a new lead singer. When the episode originally aired, Ritchie Blackmore was still performing with Rainbow; Deep Purple wouldn't reunite for another two years.
The only reason why Johnny and the others are allowed to stay in the hall is because Johnny has volunteered to run Devo's sound board after the band's tech got sick. But when Devo goes on, Johnny is nowhere near the sound board.