The BOSS has a ton of shout-outs.
- Aside from the characters literally showing up, Undertale also has a number of shout-outs within the game's dialogue.
- One of the lamps in Jack's apartment can cause him to quip "A not-so-conveniently shaped lamp!"
- Since Billy (technically the Irresponsible Dad's son) is a party member, Happy Wheels has a massive one.
- There's a possible shout-out to The Cave in the Portal section, only seen if Jack and Cliff get blown up in the holding chamber. When they respawn, GLaDOS says that they're not allowed to die because it would raise Aperture Science's insurance rates. This is almost a direct quote from the The Cave, as the Cave says this to the spelunkers should they die in the opening section of the game.
- Jenny finds a piece of paper on the wall of the Layers of Fear section which says "To spell a word correctly is to honor its meaning." This is a direct quote from Spelling Jungle, also known as Yobi's Basic Spelling Tricks.
- In the Layers of Fear section, Jenny observes that a substance on the wall looks like tar. She then refuses to touch it, noting that it's a one-hit death in Commander Keen.