Sequel episode, Robot Hell is re-visited, The Robot Devil returns, he is seen practising playing music with his band in reference to the episode's musical number.
When Fry goes to Coney Island which hasn't been touched since the 21st century (The Simpsons' time era) there is a love heart on the wall with the intials H.S. and M.B. This is Matt Groening alluding to the characters Homer Simpson and Marge (previously Marge Bouvier before marrying Homer) from his other show.