500 Keys/References

Cultural references

 * When Homer flies off in the Duff blimp he shouts down to Barney: "When we see each other again, you'll be an old man and I'll be a baby," a reference to the in Einstein's  and the movie . It is perhaps more likely to refer to Michael Moorcock's  steampunk trilogy. Homer delivers the line from a blimp/airship which features strongly in Moorcock's central character's life: 'Oswald Bastable' was in turn borrowed from E Nesbit's children's stories.
 * When Lisa finds the hidden classroom in the basement of Springfield Elementary School:
 * She says, "I've discovered a mystery wrapped in a riddle in the basement of a lousy school!" This is a reference to a speech by at the beginning of, in which he described Russia as "a riddle wrapped in a mystery inside an enigma".
 * She observes that the stage is set for a production of the play  by.
 * The objects in the classroom appear real, but on closer inspection Lisa discovers that they are fake. Near the end of 2001: A Space Odyssey, astronaut David Bowman finds himself in a similarly fabricated hotel room.
 * There is a poster in the hidden classroom featuring the element symbols from Avatar: The Last Airbender.
 * Lisa returns to the classroom with a huge crowd and school newspaper reporters in tow, only to find nothing but bookshelves, similar to the Geraldo Rivera special , where after a huge media buildup the vault was anticlimactically discovered to be empty except for dirt and debris. This could also be a reference to Grift of the Magi where Lisa brings Marge, Homer, and Chief Wiggum to the school to show them the true intentions of the company that was running the school at the time.
 * The Pooter Toot Express (the wind-up toy which Marge and Maggie find) may be a parody of and its products.
 * When Bart gets onto the window washing platform, the music playing is an of the title theme from .
 * During the blimp pursuit (Chief Wiggum and Lou in the Springfield Police Blimp chasing Homer in the Duff Blimp):
 * Homer eludes Wiggum by suddenly climbing and flying over a billboard advertising a performance titled "Zoomanity". Zoomanity was previously mentioned by Bart in the episode Marge's Son Poisoning.
 * Wiggum crashes into the billboard and cries "Oh, the zoomanity!" as his blimp catches fire, referencing the.
 * When Otto drives the bus off the bridge, he wonders aloud whether they'll fall slowly, as in the movie .

Trivia

 * The chalkboard gag is a reference to Kristen Schaal's name being misspelled as "Kristen Schall" in the closing credits of "Homer Scissorhands".
 * This is the second time an episode's chalkboard gag has referred to the immediately preceding episode. The first was in "Homer the Heretic", where the chalkboard gag "I will not defame New Orleans" referred to complaints the show received about a song about New Orleans in the previous episode, "A Streetcar Named Marge".
 * In the band room scene of the opening sequence, Lisa plays a violin instead of her saxophone.
 * A violin solo version of the theme song plays over the closing credits.
 * When Lisa first finds the hidden classroom, a hamster ball with a skeleton inside it is briefly seen. Whether it's Nibbles or another hamster isn't known.
 * The letters "RDRR" are written on the chalkboard in the hidden classroom, which is a clear reference to a classroom joke in the 1990 episode "Bart the Genius".
 * Nelson carves his trademark laugh out of wood in shop class.
 * Homer's car registration says "NABF14", the production code for this episode.
 * One of the mannequins inside Bus 23 resembles Janey.
 * The couch gag was re-used in the Season 23 episode "A Totally Fun Thing Bart Will Never Do Again".

Continuity

 * Homer has several keys from his previous jobs:
 * Homer the Smithers ("Homer the Smithers")
 * Bounty Hunter ("Sex, Pies and Idiot Scrapes")
 * Mr. Plow ("Mr. Plow")
 * NASA Space Shuttle ("Deep Space Homer")
 * Midnight Towboy ("Midnight Towboy")

Goofs

 * The track of Bus 23 can be seen going into the river, but when Lisa falls there is also a track on the other side.
 * However, this could be explained by the bus first crossing the ice bridge, and then falling into the river backwards.
 * Also, from the way Superintendent Chalmers refers to the ice bridge, it's been used before. Previous vehicles (maybe even buses) crossing the bridge successfully would explain tracks on both sides of the river.
 * When The Pooter Toot Express nears the river:
 * 1) It can be seen going down a short hill, which wasn't there when the blimp flew over the track.
 * 2) The track of Bus 23 is no longer visible, when it was clearly there before.
 * 3) It crashes into a tree that wasn't there before.