Warning: Spoilers Off applies to these pages. Proceed at your own risk.
General
In Underverse 0.2. Having run out of options and with Cross distracted, Sans decides he has no choice but to use his SPECIAL ATTACK. It's literally just him throwing his slipper at Cross. Everyone, even X-Event Chara, can only stare at him in complete disbelief.
Ink: Your slipper. Your special attack. Is your slipper.
The entire phone call Underfell Sans has with his "boss".
Made funnier afterwards when Underfell Papyrus realizes that Underfell Frisk thinks his "chance"note Giving him 10 minutes to hide for the rest of his life or else he'd use his skull to serve his spaghetti if he ever found him. is too mean. His response?
Undernovela in general. It's pretty much a Mexican soap opera, and Error is just so into it.
Error: Go, Asgoro! Save your child man! Ink: Huh. This is getting interesting. Error: I KNOW, RIGHT!?
Cross and X Chara's clothes inexplicably change to fit Undernovela. And Cross is breaking out in Spanish for no reason. Chara's face sells it.
X Chara refusing to take half of Novela Frisk's soul for the sole reason that they have a mustache.
And poor Error's reaction when he sees that "those anomalies" have invaded his favourite show.
During Error's dream sequence(?) when Ink teleports into Undernovela while Error's watching, in the background you can see Classic Sans with quite a large band-aid stuck over the spot where X-Event Chara struck Sans and his ketchup bottle in Underverse 0.3 part 2.
Fell: This is bullSHOES (accompanied by a duck sound effect). What was that sound? What the FUNK!? *growl* BEACH! RADhole! MUSTard! Suck my DESK! FUNK you! ShED! MotherFUNNY! DUNKhead!
In Underverse 0.4, we're introduced to Outertale Sans by having him pull the classic "Whoopee cushion in the Hand" trick on Classic Sans, but Classic Sans also has the same trick, thus leading to a 15-second-long, and very LOUD Whoopee cushion fart. And we do mean LOUD. So much so, it ruins the ambiance andEVERYONEfrom Outertale can hear it.
Jakei's 2019 April Fools' Dayvideo. In it, XGaster happily announces that he is pregnant with a baby boy, who is shown in the next scene eating moss alongside a disgusted Chara and intrigued Frisk, all but confirming he is the X-Tale version of Kris.
In X-Tale VIII, we learn of how Mettaton was created. Once Mettaton wakes up, they immediately start chewing on the cables holding them up.
After a break, we cut to see Chara/Frisk struggling to get their HEART LOCKET out of Mettatons mouth.
Underverse 0.5: The final fate of Sans's old hoodie: unceremoniously tossed in the pyre by Papyrus, while Undyne yells in glee, with a shocked Sans unable to stop it.
Asgore tries to cheer on Frisk by invoking Toriel's name in front of her. Her death glare, followed by Asgore's sheepish retreat, is a sight to behold.
Error gets batted into a pile of plushies... that turn out to be explosives!
Underverse 0.7 Part 2:
Aftertale!Papyrus' shocked face at seeing a third Sans, aka Cross, is a glory to behold.
Even after being pushed over to complete nihilism, Classic!Sans can still crack a snark when he asks Ink if Ink changed his clothing style after Sans already changed his.
Other
XGaster in the main series? Terrifying, currently has the biggest contender for most evil person in the series. XGaster in the official blog?Absolutelyhilarious. Jakei has a tendency to draw him with a ":3" expression on his face, adding to the hilarity.
Even better? According to a moment in this comic, it's his canon friendly face.
The creator of Undertale fan-comic "Epictale" made a comic of all the characters of XTale meeting their Epictale counterparts (A nod to the two fan works' Friendly Fandoms). XFrisk and Epic Frisk are both being snarky to each other, Epic Chara is amazed at XChara's height, Epic Undyne gives XChara, Epic Alphys, and even XAlphys a Horny Nosebleed, both Papyrus's are amazed at the situation, Epic Mettaton's only reaction to XMettation is "Who made you and why?", XToriel and XAsgore can only comment on their Epictale counterparts' larger ears/horns, and predictably, XGaster and Epic Gaster hate each other (Leading to Cross and Epic Sans to not that this was a bad idea). Funny enough, but then Jakei made a response comic, leading to a trilogy of the characters playing a Try Not to Laugh game.
Cross calls the game "The Activity that will provide Entertainment in this Social Gathering", rather than just a "party game.
XChara tries to tell clever jokes to Epic Gaster and Epic Chara, who both refuse. He then yells out "Dude, your ████ is hanging out!" This disqualifies XFrisk. XChara himself then gets disqualified for just casually laughing.
Epic Mettaton gets XMettaton to laugh by bribing him with a bag of marshmallows.
Epic Chara is sandwich between the still fighting XGaster and Epic Gaster, much to their discomfot.
Epic Chara: He-Hey guys... wanna hear a joke? Epic Gaster: Is it about [XGaster's] unperfect universe?
Epic Frisk tries to disqualify Epic Toriel by pulling the classic 'detachable thumb' trick on her. In an ironic twist, they then get disqualified for saying "Haha, got you~"
Cross: "Haha", indeed.
Epic Toriel and Epic Asgore threaten to pull XUndyne and Epic Undyne's fingers if they don't laugh on command. They decide they'd rather preserve their dignity than win the game.
XGaster and Epic Gaster snap and threaten to destroy each other's universes using their respective weapons. Epic Chara then tries to stop them.
Epic Chara: I-I, uh, s-s... s-stop fighting or I will tear you to pieces, uhh... I-I don't plan to hold... b-back. XGaster and Epic Gaster: You? Ha! Epic Sans:Aaand the two boomers are out! Thaaank heavens, bruh!!
XAsgore and Epic Toriel get into a Dad joke + Pun war. The cringe that is being flung around by the two is so powerful it starts to cause earthquakes, until the two say the exact same cringe joke, which causes part of the planet to explode.
As part of X!Gaster's routine, at exactly 4:10 PM, he stares out the window to find Epic!Gaster sitting on the rail. They enter a glaring competition, and it's implied this lasts for 4 hours.
. No words can do it justice.