They are seen separately afterwards among many other less notable Phineas and Ferb Background characters
Among the many objects that fall from the roof when the background screen collapses are the cat, the two elders playing checkers and the guy with a poodle who were also disturbed when the football field was ripped up by a drill in Rooting for the Enemy
The performers who join together for the impromptu finale includes all 6 actors (7 including Milo) seen on stage, two actors who were startled by the fire extinguisher back stage, and the man in the giraffe costume who ran across the stage earlier, leaving only two people previously unseen in the episode.
"Smooth Opera-tor" was one of the first episodes of the series to premiere on Disney Channel before airing on Disney XD.
During the performance the positions of the audience around Amanda and Melissa changes. Most notably, in some shots Milo and Zack's vacant seats have been taken