During the vessel creation scene, giving yourself the same name as your vessel results in the voice dryly commenting on how on-the-nose that is.
If you only talk to Temmie after you're partnered with Susie and not before, then you will have never seen her boiled egg "partner". As such, her example of Susie being mean changes from saying her egg would never hatch to making fun of the way she talks.
When you're first introduced to the overworld dodging mechanics, if you keep leaving and going back into the room to get hit by the projectiles until your HP is below a certain number, the game will comment on it at the save point in the next room, saying that you "punished yourself" for some reason.
If you select the FIGHT command while Ralsei is training you, but keep intentionally missing by not hitting the button to trigger an attack, Ralsei gets increasingly confused and agitated, and eventually decides to move on to teaching you how to DEFEND. If you then select "FIGHT" and miss again, Ralsei reaches his breaking point and politely ends the lesson.
K. Round isn't meant to be beatable by force, as it can heal faster than you can wear its health down. It still has "low health" in-battle text and a rudimentary defeat animation, just in case you tamper with the game to out-damage its healing.
If the player goes back to Rouxls Kaard's shop after the K. Round rematch, he has new dialogue praising the heroes.
Originally, in the SURVEY_PROGRAM version, getting to the Dark World in under eight minutes allowed the player to get an item that skipped cutscene dialogue, on the off chance the player was speedrunning the game. The 2021 version removes this item and just lets you skip text from the beginning.
Backtracking when Susie is or isn't in your party prompts different dialogue from NPCs.
If you backtrack after finding Susie in the Dark World for the first time, she leaves Kris behind, saying that they have no idea where to go.
Passing over Lancer in the Field makes him ask whether you're going to say hi. If you then return, he says that saying hi won't stop you from getting thrashed by Mooks.
Returning to Seam after opening the door, but before fighting Jevil replaces their hint with anticipation over the coming battle.
Trying to inspect the sink in Rudy's hospital room before talking to him will have him ask Kris if the sink is more interesting than him, pretending to be hurt by such a thing. Trying to inspect it a second time will just have Rudy tell Kris to come over and talk to him instead.
Chapter 2
If you didn't talk to Alphys in the Chapter 1 epilogue, she'll express relief that Kris is okay instead of remarking that Susie arrived before they did.
The initial armor for Noelle is a watch, which is possible to remove from Noelle and equip on Kris. Upon getting back to the Light World on a No Mercy run, Noelle tries to convince herself that what happened in the Dark World was All Just a Dream. If you still have Noelle's watch equipped on Kris when talking to Noelle in the hospital, she'll notice and realize theimplications.
Normally, Noelle has SnowGrave for only one fight on the No Mercy route, after which she leaves. However, before the 1.05 patch, by sparing Berdly and quitting the route just before point of no return, Noelle would keep the spell, while the player could still encounter an enemy (via a scripted encounter in the cheese maze that doesn't appear on No Mercy route but comes back if you quit it). The enemies have a unique animation to getting hit with it. They visibly die from it too, rather than freezing or running away, as they now fade into a blood red mist. This mist death animation is only used one other time in the game, by the Weird Route exclusive Pipis enemy - Which is notably the only enemy that you can actually kill instead of it running away.
Though she leaves your party before you can get them, Noelle has comments for drinking the Susie and Ralsei flavored tea (she really likes the former, and the latter's tea is apparently invisible to her, which has… implications).
If you ask Sans about his brother without having talked to him in the previous chapter, he'll say that he doesn't have a brother at first, before questioning how you'd even know he had a brother if he's never mentioned him to you before.
When Kris is carrying the dusty orb of junk on their head, if you go down the hall, you can encounter Alphys talking to Toriel about Kris. It's possible to call Toriel on your cellphone at this point, which has unique dialogue that varies based on whether or not you'd overheard their conversation before calling. This is a double whammy because most people don't think to go down the hallway, never mind call Toriel at the same time (since Kris's phone never even gets mentioned in the chapter).
Rudolph Holiday has unique dialogue if you go to Sans's store and initiate the interaction between Sans, Toriel, and Asgore beforehand and then go talk to him. He'll get jealous that Sans is giving Asgore free pickles and threaten to turn Sans into "a goddamn xylophone".
Attempting to interact with the sides of Queen's arcade machine will fail.
(You can't pro game while facing this direction.)
If you hacked the game to give yourself the unused, yet fully functional Trefoil weapon for Kris in Chapter 1, in Chapter 2 if you attempt to have Noelle equip it, she'll have a unique descriptive response that brings up the fact that it's unused.
In order to beat Sweet Cap'N Cakes, you have to have Kris dance with an ACT command for four turns before Susie gets fed up and unlocks S-Actions for herself and R-Actions for Ralsei. If Kris is knocked out before the four turns pass, it causes Susie to question why she and Ralsei can't ACT without Kris, unlocking the actions earlier.
If you rush ahead at the part where Susie and Ralsei stop to watch fireworks on the conveyor belt, they will catch up to Kris by shortcutting across invisible floor tiles (the same kind that's sometimes seen in Undertale as an impromptu solution to NPC pathing problems in that game).
In the epilogue, checking on Lancer in your pocket while you're in the same room as him changes his line to mention that he's diving out of and into your pocket quickly.
After the Berdly fight in the city, Noelle will say with a mix of shock and frustration "He hit me in the face with a tornado". However, if she never took damage during the fight, she won't say this line. She also won't snap at Berdly for attacking her in the battle itself until the game chooses her to be the one to take damage.
It is possible to completely fill your Weapons/Armors inventory if you go out of your way and waste quite a bit of money on dozens of duplicate items. If you do this, and then defeat Spamton NEO, Spamton (the item) will specifically comment on exactly how many item slots you would've had to waste, get angry at you, and leave in frustration. The item will then be left in a chest on your way out.
If Ralsei defends on the first turn of Queen's fight, he'll poke his head out of his scarf to deliver his line.
Having a full weapon inventory when trying to buy the Thorn Ring will have Spamton comment on how many weapons you are able to hold... and how you've filled it all up.
Spamton: "WOW, NICE AND SPACIOUS!! PERFECT FOR [City Livin']
HOW CAN YOU [Afford] THIS!?!?! ... YET THERE'S NO ROOM FOR A [Ring]
THERE'S A [Boy], BUT THERE'S NO ROOM FOR A [Ring]"
Likewise, even though you're guaranteed to have enough Dark Dollars to purchase the Thorn Ring once it becomes available, Spamton has special dialogue for if you modify the game to have less than $1997 on-hand, complete with a Japanese translation.
Spamton: "[Money] NO"
If you fight Spamton NEO on the Weird Route with a filled weapon inventory, his PuppetScarf will spawn in a chest in the castle town.
If you backtrack all the way to the beginning of Cyber World late into the game and save at the very first checkpoint again, the dialogue text will be different, making fun of the player for their "pointless backtracking" (since you don't get anything for it).
In the first Annoying Mouse Room, an attentive player may notice they can place a block right above where Noelle will be standing when they press the button for the first time. If they do, it'll redirect the mice away from her... straight into a traffic cone which they'll bounce off of, causing them to hit into her anyway.
In the second Annoying Mouse Room, it's possible to trap the mice in an infinite loop, in which case the mice will disappear after about 5 seconds.
The ringing phone in Spamton's shop is actually an event set to the song playing in the background ("Dialtone"), so removing the song from the shop will cause the phone to stop ringing. A similar thing happens in Rouxls Kaard's battle, where removing the song prevents Rouxls from saying "Hey" (in a speech bubble), and in Queen's Mansion, where removing the background music stops the paintings from laughing.
Before the fight with Spamton NEO on the Weird Route, there is a vending machine where you can buy CD Bagels (since you can't backtrack to other shops at this point). If you somehow don't have enough money to use it (or you keep using it until you run out of money), the game will have some unique dialogue making fun of the player for wasting the huge amount of money they earned:
You didn't have enough money. Which, is surprising.
If one tried inspecting the sink before talking to Rudy in Chapter 1, doing so again in this Chapter will have him once again be playfully hurt that Kris went to the sink first instead of him again, even keeping count by saying sink - 2, Rudy - 0.
, and eventually decides to move on to teaching you how to DEFEND. If you then select "FIGHT" and miss again, Ralsei reaches his breaking point and politely ends the lesson.