Due to the game's multiple endings and self-referential sense of humor, there are a large number of Brick Jokes in Undertale.
- During the Golden Ending, Toriel will remark on whether you flirted with her, called her "mother", or both (as well as which order you did them in, if you did both).
- In the Ruins, one of the Froggits asks if you find the yellow names (which indicate when a monster is spareable) useful. Saying no and talking to the Froggit again will have them say that the yellow names were thrown away. Later on, when looking through the piles of trash at the dump in Waterfall, the narration will mention that there are yellow names there.
- Checking out Papyrus's racecar bed during the hangout/"date" has him mention that he'd like to drive down a long highway if he ever reaches the surface. He does just that in the best ending.
- At one point within Sans and Papyrus's puzzle hijinks, Papyrus tells Sans he's frustrated about Sans napping all night, at which, Sans corrects him by saying, "i think that's called... sleeping." Later on, a certain phone call with Papyrus and Undyne involves Papyrus saying "LATELY [SANS]'S BEEN NAPPING OVER 7 HOURS A NIGHT...", which the latter then responds with, "... wait, isn't that just called sleeping?" (This phone call also reveals that Papyrus himself rarely sleeps, in spite of having a bed himself.)
- One of Papyrus's puzzles, designed by Dr. Alphys, has a ludicrously complicated set of rules... complicated enough that even Papyrus can't keep track of them due to her poor handwriting, and if you say that you didn't understand the rules, he tries repeating them, but gets them mixed up and gives up on the puzzle entirely. Around two and a half areas later, all the way in Hotland, the puzzle returns, and you're asked to remember them all, and the only practice you could have gotten was on an… underwhelming "randomly-generated" puzzle which always ends in the same outcome. Thankfully, you're not expected to succeed in finishing it within the given time limit, although it's still possible. Also, though you wouldn't know this on your first playthrough, the machine which Papyrus uses to set up the puzzle is Mettaton, making it likely that the "randomly generated" puzzle's failure was just a prank played by Mettaton on Papyrus.
- Near the beginning of the game, the player encounters a spider bake sale. Players can put money in a spiderweb and a spider will bring them a donut or some cider. It's a cute and original way to obtain healing items before encountering the first town. In the second-to-last area in the entire game, the player meets more spiders who are hosting a bake sale, who are none too pleased if the player has not supported their enterprise. The player will have to survive an encounter with Muffet, their leader. If the player has bought a donut or cider earlier and consumes it during the encounter with Muffet, the "Spare" option will immediately become available. Lampshaded in the Genocide route, where Muffet will simply spare you because you haven't killed any spiders up to that point. Read: There are no spider enemies besides Muffet, and even if you're on a genocidal killing spree, you can only kill anything that appears in a fight scene.
- During his boss battle, Papyrus boasts that "THE KING WILL TRIM A HEDGE IN THE SHAPE OF [HIS] SMILE!!!". In the True Pacifist ending, this is exactly what happens.
- Also in one of his call queries, Papyrus wonders why they haven't come up with cool inventions like "PHONES THAT CAN TURN INTO JETPACKS". Guess what Alphys adds to your phone when she upgrades it?
- At one point during Undyne's chase against you, Papyrus will call, stating you, he, and Undyne should get together sometime. Calling Papyrus within the same area after befriending Undyne will result in Undyne asking Papyrus what he called you about. Cue a bewildered reaction from Undyne.
- In the playable epilogue, Toriel's pie in her kitchen seems to have been eaten by dogs. At the end of "Hard Mode", you find it was the Annoying Dog who ate it.
- Also in the epilogue, the mice will have finally gotten some cheese (or spaghetti). All four of them.
- When first dealing with Papyrus, he has a trap about trying to make you eternally eat spaghetti (which would fail anyway since it's frozen). He will casually mention he might eat it himself afterwards. When walking back during the playable epilogue, you will find that Papyrus or the mouse indeed somehow had heated it up and eaten some of it.
- On the way to Snowdin, you can encounter a Snowman who wants to see the world. If you carry a part of it with you through the final neutral boss and have not killed Papyrus, Sans will congratulate you. In the Golden Ending, if you return to the snowman with his piece, he will thank you, and ask you to keep holding onto it outside the underground.
- The married dogs you meet in Snowdin mention placing second in the '98 Nose Nuzzle Championships. Asgore has the trophy for placing first. Also, if you complete a True Pacifist Ending and don't use the Stick to spare them, their yellow credit text says "Finally Nose Nuzzle Champions."
- Monster Kid mentions how cool it would be for Undyne to show up at his school and beat up all the teachers, but resigns himself to the impossibility that Undyne would hurt anyone innocent. If the player calls Undyne while outside the school in Hotland, she excitedly offers to beat up the teachers as a way to make school more interesting. And if you call her again in the same place a second time, she will also say that it would be too mean. And in the Neutral-Pacifist ending where everyone but Asgore lives, she herself winds up becoming a teacher at Toriel's school.
- On the Steam page for the game, it asks you to count the amount of dogs in the game. When you encounter Endogeny in the True Lab, the text says "It's unclear how many dogs this counts as."
- If you examine the plants in Toriel's house after reading the book on the shelf in her room, the narration will identify the cattail plants as a "Water Sausage". Shortly after leaving the Ruins, you can examine Sans' first sentry post and find bottles of ketchup, mustard, and relish inside it. Way later in Hotland, we get a 2-for-1 payoff: Sans is actually selling Hot Dogs at his sentry station here... but if you buy one and examine it, you'll see that he just pranked you and it's a Water Sausage and not actual meat. However, the item will still heal you.
- Early on, Toriel asks you if you've got any allergies. As it turns out, this concern is entirely justified, as the human is allergic to Temmies.
- In Snowdin, you may encounter a large wolf-like monster taking huge blocks of ice off a conveyor belt and throwing them into the river that goes all the way to the beginning of Waterfall. Later on, while being chased by Undyne, you'll see these same blocks flowing below. Finally, you'll see them dropping into the CORE, ostensibly as cooling or for making steam. And when you return to Snowdin in the Playable Epilogue and talk to him, he's glad that he's finally able to take a break from his job and go buy some pants.
- A while after you reach Snowdin during a Genocide run, Sans will warn you that "if you keep going the way you are now... you're gonna have a bad time." Sure enough, if you get to Sans's boss fight and survive his first attack, the flavour text reads "You feel like you're going to have a bad time."
- Examining a cactus in Toriel's house has the text call it "The most tsundere of plants". The only other cactus in the game is in Hotland, where the Tsunderplane can be encountered. It gets a second payoff in the Playable Epilogue of the Golden Ending.It's not like this cactus was waiting for you to come back, or anything...
- If you decide to explore the upper floor of Alphys's lab, you'll find a machine that has what appears to be grass sticking out of the top, and a tube dripping pink goo into a bucket from one side of the machine. At the end of your "date" with Alphys, we learn what exactly that machine was and what its contents were; when she confesses everything to Undyne, the first confession is "I told you that seaweed was like...scientifically important...Really, I just...use it to make ice cream!" In a phone call with Undyne, it is also revealed that on several occasions, the only reason she didn't kill you on the spot was that Alphys had lied to her about the importance of the seaweed.
- If you wait in the Ruins when Toriel tells you to, she'll keep calling you until a dog steals her phone. An optional puzzle in Waterfall leads to a room where the dog somehow ends up in your inventory, and if you try to call Toriel, it will turn out that the dog stole her phone again.
