Harry Potter: Sideshow Bob says that his plan to kill Bart will make it the greatest murder since Snape killed Dumbledore, which happened in the sixth book, Harry Potter and the Half-Blood Prince. Sideshow Bob also is not insulting the book when he says, "It's a four-year-old book." He is actually correct. Harry Potter and the Half-Blood Prince was released in July 2005, four years and ten months before the episode aired. It would have been more correct for Bob to refer to it as a five-year-old book; however, when the script was written, it could have actually been closer to four years than five and the writers forgot to change it.
The song that Bart and Milhouse sing (when they try to bait "Walt" into exposing himself as being Bob in disguise) is from the Gilbert and Sullivan play The Mikado. When Sideshow Bob reveals himself to Bart, he then sings part of a song from the same play. Gilbert and Sullivan were first mentioned in an earlier episode Cape Feare.
The episode's plot is based on the movie Face/Off. The movie's title is punned in the episode "Faith Off."
The stuff the cops from New Jersey say after handcuffing Bob is a reference to The Jersey Shore.
The ending scene, where a cousin of Ned Flanders arrives and appears to be just like him (including a bit of similarity in appearance) is likely a reference to the TV series The Patty Duke Show, which dealt with two cousins who looked exactly alike.
One of the magazines Bart looks through in Walt's house is called "Not MAD Magazine," which is a reference to the popular MAD magazine, even having the same kind of cover design.
Adding to the fun, mystery, and controversy regarding Springfield's State: The cops who arrest Sideshow Bob at The Five Corners seem to be from New Jersey, judging from their accents and cultural references, with one of them parodying the speech patterns and expressions of comedian Andrew Dice Clay.
The characters that Marge and Homer list off to Bart as having the same voice as Sideshow Bob are all played by Kelsey Grammer, who voices Bob in the series.
Previously, Frasier Crane, a character played by Grammer, appeared in a Cheers-like bar in "Fear of Flying". Frasier did not speak in that episode because Grammer also voiced Sideshow Bob.
Bart's graffiti name El Barto appears on the prison wall that Bob climbs over when escaping from prison.
When Bart hears Bob as Walt say "Hello, Bart" again after asking him to repeat the phrase, a montage of the times that Sideshow Bob says that particular phrase was shown with the first three being from the episodes "The Great Louse Detective", "Sideshow Bob Roberts" and "Brother from Another Series". The "Hello Bart."s from Cape Feare, Day of the Jackanapes, and the other from The Great Louse Detective are the only ones not shown as the remaining two in the montage were made for the episode (the latter was actually referring to the acronym for the Bay Area Rapid Transit, "BART").
When Homer is signing the radon disclosure for the new house, the date on the paper is shown to be May 16, 2010, the same day this episode premiered.
The signs held by the Icelandic protestors are all very grammatically incorrect.
"Við vilja okkar pening!" - should be "Við viljum peningana okkar!" which means "We want our money!"
"Neitun Homer!" - Directly translated it means "Denial Homer!"
"Segna upp nú!" - Segna is not a word, "segja" is, it could mean "Segjið upp núna!" which means "Quit your job now!"
Following the episode, an image of the protest in the Simpsons with a headline "Iceland in The Simpsons" appeared on the front page of "Fréttablaðið", Iceland's most widely-read newspaper. They also dedicated about half a page to The Simpsons. It should be noted that The Simpsons was not criticized in the newspaper and that The Simpsons are very popular in Iceland.
Iceland has been referenced twice before on The Simpsons, once Carl mentioned that he grew up in Iceland and in Worst Episode Ever there was a food stand in the Springfield Squidport named "What they eat in Iceland".
"Beowulf" is Old English.
Near the beginning of the episode, Walt Warren buys Ruth Power's former house, and Lisa mentions that Ruth moved to Detroit. But at Walt's party a few minutes later, Ruth can be seen in the crowd.
In the car, Bart's mouth is taped, but after Sideshow Bob's backstory his mouth is untaped. Again, Bob retapes it and again in the next scene, it is untaped.
When the real Walt Warren appears (with Sideshow Bob's face) and Marge and Homer catch him, Homer ties him up with Flanders' garden hose. Even though the water was running before, when Homer ties up Walt, there's no water stacking up in the taut hose, nor is there any water coming out.
When the waitress rips Walt's face from Sideshow Bob, Bob holds it in place with a belt. Walt does the same thing when Bob's face is ripped from him. Later in the episode, however, their faces stay in place in spite of the fact that they aren't using anything to hold them on. It is implied that they sewed them back on offscreen, due to Walt getting distracted by a bee being stuck underneath his face.
At Five Corners, when Chief Wiggum explains how the Springfield cops searched for Sideshow Bob, Bob tosses Bart and Walt into different states. Moments later when a second state's police force arrives, Bart and Walt are in the same state again.
At the party when everyone is trying out the Wifi Jessica Lovejoy can be seen next to the Reverend but she has Helen's colors.
Bob tells Bart that the term epitaph refers to your last words when in actuality it refers to the writing on your gravestone.