During that Agatha reveal song she said "And I killed Sparky too" literally the instant after I said "she killed Sparky too" in my head.

They must have completely audience tested the show while in production to make little tweaks. So many subtle things like that (for example writing on the whiteboard "why sitcoms? why a hex shape?) They are breaking the "5th" wall at this point - not just talking to the audience but reacting to the audience's responses in real time.