Lisa claims that tenor Andrea Bocelli recorded an entire song on his back. Bocelli apparently did record one track on his Amore album on his back, and demonstrated this technique on an episode of American Idol.
The scene of Lenny and Carl watching Homer's opera performance from the rafters is a reference to a similar scene in Citizen Kane.
During the opera montage, Swan Lake, "Op.20: Act I- No.5 Pas de deux: I Tempo di valse ma non troppo vivo, quasi moderato" by Pyotr Ilyich Tchaikovsky plays.
Homer's entourage, Lenny and Carl, is a parody of the HBO series Entourage. The music when Lenny (who dresses like Turtle) and Carl are in the limousine is the show's theme song, "Superhero" by Jane's Addiction.
When Carl and Lenny are sent away from the table at The Gilded Truffle, Carl mutters "Yoko", at Marge. It is a reference to Yoko "breaking up The Beatles", but in this case Marge is breaking up the entourage.
When Julia takes her helmet off to reveal that she's a female biker, Homer asks if this is The Twilight Zone.
Restaurants spotted or mentioned after the family leaves church: Griddler on the Roof, Thank God It's Fried Eggs, Luftwaffle's, Bodacious Frittatas, Buffet the Hunger Slayer, and Denny's.
When Homer forgets his lines he sings "Uh-oh Spaghetti-os"? This was the catch phrase of the fictional police officer Homer Simpson in the season 10 episode "Homer to the Max".
When Burns and Smithers are "shopping" at the morgue, a sign on the wall says "HOSPTIAL MORGUE".
Carl and Lenny are shown in the rafters commenting about how their "seats are terrible." However, after the performance ends, Carl and Lenny can be seen in the audience as they give Homer a standing ovation (top right quadrant).
Rodolfo is referred to as "Rudolfo" on the translation during La bohème.