Midway through the episode Brows Held High reviewing Vase de Noces, Kyle gleefully squees over the baby piglets before things take a dark turn.
Kyle: Awwww! They die. (Beat) They die because [the protagonist] kills them. (Beat) He hangs them. In little pig-sized nooses, in a row like convicts.
Derrick Comedy takes this to extreme levels in its Keyboard Kid sketch. Jeffrey the 6th grader is performing with his "amazing keyboard" at the 6th grade talent show. He starts out cheerfully dancing to the beats and vocal samples of his sampler keyboard...
My name is Jeffrey! Cool! Alright! C-c-cool! Alright! Get down! Check it out! Cool! Alright! C-c-cool! Alright! G-g-g-get down! Kick it! If I mess up my dad's gonna beat me!silence
Waverly Films does this in "Wanna buy a ghost?". The mood changes from serious and concealed to a common reaction.
Shady Guy: Hey. WANNA BUY A GHOST?
Tourist: A GHOST?
Shady Guy: SHHH. KEEP IT DOWN.
Tourist: How much?
Shady Guy: 5,000 DOLLARS.
ThisCyanide&Happiness short does this over the course of 30 Seconds
The "Child Molester" clip from AMV Hell 4. It's an hour of silly jokes and you've long come to expect Black Comedy, but this comes completely out of the left field. There's no joke in it. It's just there, making you feel terribly akward. Possibly intentional.
Doctor. Flipping. Horrible. Not only does the tone do a complete 180 from lighthearted comedy to straight-up tragedy in the space of 45 minutes, during the most heartbreaking moment of the entire series, the Really Dead Montage shortly after Penny dies, one of the newspaper headlines which flash across the screen says, "COMMUNITY MOURNS DEATH OF WHAT'S-HER-NAME". And this is during a dramatic scene.
In that same Really Dead Montage, we have "CAPTAIN HAMMER DEFEATED: Hero 'Unavailable for Comment' For First Time Ever."
And the last two seconds of the entire thing go with an instant cut, within a song, from triumphant Dr. Horrible entering the ELE boardroom to absolutely emotionally destroyed Billy
Not that the earlier parts of the series are immune to this: the Act II song "My Eyes" features Dr. Horrible spying on Penny and Captain Hammer's dates via some extremely transparent disguises (including one use of Mobile Shrubbery). Pretty funny...and then you hear what he's singing:
There's also the character of Milo Taylor. For most of his life (lasting up to the exact halfway point of the game), he was basically Refuge in Audacity personified with a dash of Crazy Awesome for flavor. Then he got thousands of volts sent through his body and his collar exploded, rendering him unable to even scream because his trachea was torn apart. Then he bled to death very quickly afterwards.
There she is!!! from http://www.sambakza.net/. The first three episodes are pretty light-hearted, but the fourth episode, "Step 4 - Paradise" takes on a darker mood. This shift was likely alluded to at the end of "Step 3 - Doki and Nabi" when a rock gets thrown through Nabi's window.
Specifically, in the fourth episode, Doki and Nabi are subjected to constant persecution because of their, erm, mixed dating. Doki gets injured by an angry mob, and one of her pets is actually killed, though it was probably an accident.
Then, in this series' second instance of mood whiplash, the fifth and final episode goes back to being cheerful and action-oriented, but is still more serious in tone than the first three. Incidentally, if you've seen the whole series, then the street-punk rabbit handing Nabi the plane ticket probably qualifies for a Crowning Moment of Heartwarming.
The D&D PHB PSA series on Youtube does this, interspersing the comedic interviews with D&D characters with the lonelymal69 subseries, where Villain Protagonist Malcanthet the Succubus Queen attempts to avoid being executed by Order of Saint Cuthbert. And the lonelymal series itself does this, mixing scenes of drama with ones where the demon lord of the air Pazuzu attempts to prove to Mal that chickens are the deadliest creatures on Earth
Happens in Alexander Leon's Mario Brothers, especially the last one, where Mario effectively kills himself by letting a flowing pool of lava burn him to death, after nearly everyone else and their mothers were already dead... and then cut to the happy underwater level music for the credits.
This Very Wiki in general. The pages here vary from lighthearted and silly to dark and depressing. It's easy to click a potholed link, and end up on a page where the tone is the complete opposite from the one you were previously at.
The So Bad Its Horrible page. At one moment, you'll be giggling at the snarky descriptions of hilariously bad attempts at entertainment or truly pathetic fanfiction, and at the next moment reading about the dangers of hazardous toys and rides, and the harm those things have induced on innocent kids.
The Real Life section of Headscratchers: ranging from an ardent discussions of cultural perceptions to vehement attacks against political beliefs to injokes to a discussion about how awesome boobs are.
A good number of screamers are like this. One moment, you're looking at peaceful, beautiful scenery as calm music plays, the next...RAAAAAAAAAAAAAAAAAAAAAAHHH!!!
Nitro Game Injection is usually a fun, lighthearted video game music podcast, but during a segment of the eight anniversary episode, that was put on hold as former show co-host Akumu returned to pay tribute to Fumapero, a doujin music arranger who had been missing and displaced after the devastating earthquake in Sendai, Japan. Luckily, she announced a few days later that she was okay and had returned home.
Happened again when Akumu, who was a close friend of recently deceased (as of 2011) game composer Ryu Umemoto, was brought back to discuss Umemoto-san's life and career as a tribute to him.
Author's Note: each chapter of the Monster Mash has its own tone, and while none of them are truly grim, some are darker than others. If you're reading this chapter right after 'The Big Battle', consider yourself warned.
DYE's Fantasy is great on this trope. The beginning is so innocent...Oh, just a bunch of teenagers doing some mischief at a school swimming pool at night and discovering love and fighting their insecurities and..WAIT, WTF IS THIS!!?!? Couple making out at the pool become a pair of disgusting alien-zombie like things. Boy hitting on the shy girl gets killed with the most painful oral sex ever thought. Shy girl desperately tries to escape, while the previous boy's dying body merges with the monster girl, and keeps squirming like an insect at her mouth. Shy girls avoid the same fate by the monster boy by jumping on the pool, which, by the way, is a door to another dimension. There, her mind is fried and she burns from the inside upon seeing a gargantuan Cthulhu-like beast who is probably the cause of all this. Video ends. and it all started like an ordinary High School story...
The BriTANick video, "The Kiss", is a rather extreme case of this trope. It goes from a good, wholesome silent film style romp to an incredibly dark and twisted take on A Clockwork Orange — All in a spine-shatteringly quick 5 seconds.
Channel Awesome contributor Spoony has a video series known as 'Counter Monkey,' involving tales from various roleplaying game situations he has experienced. In an upload concerning the Thieves' World game setting (part two, currently titled "The Chicago Way"), his discussion of the game's events involve the immortal and severely upset Tempus Thales going over the Moral Event Horizon in a severely Squicktastic fashion... and right in the middle of that topic, Spoony suddenly comments on his dog Oreo having a dream on the bed and how cute she looks when she's confused.
Dei-Sama's Shinukoto! starts out as a cute and comedic anime short about a girl trying and hilariously failing to off herself with various means, including trying to shoot herself, throwing a toaster into the tub, trying to hang herself, trying to drown herself, and finally sticking her head into an oven. It ends with a heartbreaking hospital scene, with sad piano playing and a card from some kid saying "please return," as the girl closes her eyes and the heart monitor flatlines.
Atop the Fourth Wall has one rather jarring Whole Episode Flashback, wherein the first 9/10 of it have nothing to do with the framing story, being a humorous standard lighthearted internet review of a wholesome, innocent Archie christmas issue. When the person telling the story is finally asked where he was during all this, we find out his son had died, his wife left him, and he'd spent the past half year standing outside the cemetary and doing little else, and shortly after the review we saw Linkara had found him lying in a gutter and helped him get back on his feet and get his life back together.
Also happens in "The Next 15 Screw-Ups", less intentionally. Linkara, in a quiet and regretful voice, apologises for being insensitive about depression in a previous review. Except the various items in screw-ups videos are separated by a specific comedy song running over Ren and Stimpy clips.
The Lizzie Bennet Diaries: all videos of Lydia's spinoff vlog end with an upbeat "duh nuh nuh nuh nuh nuh, nuh nuh nuh nuh nuh nuh YEAH!" by her. This becomes more and more jarring as Cerebus Syndrome sets in with the plotline revolving around George Wickham emotionally abusing and manipulating Lydia.
"To This Day" starts with a humorous anecdote about pork chops, but suddenly takes a left turn as it veers into highly depressing subject matter.
"Blue Patches" from Demo Reel. At first it starts off fairly easy with Rebecca and Tacoma finding out that Donnie's Dark Secret was that he was a ridiculously bad child actor, and Donnie being "looked after" by a kooky family. You start to relax, thinking the depressing "Lost In Translation" and "The Blair Witch Hangover" were just leading up to a comedic reveal... but then everyone finds out that Donnie's mom committed suicide when he was young.
One Girl, One Sandwich. It starts off with a pleasant atmosphere: a girl wakes up, craving a sandwich. She gets bathed and dressed before hopping in her car and driving to a WubWay. She orders a sandwich, sits down to eat it... and then the sandwich comes to life and bites her head off before attacking the camera.
Worm has a few such moments, but a particularly vivid (and spoilerish) example comes here: the chapter goes from lighthearted drama (Weaver making an emergency phone call to Glenn) to comedy (the Chicago Wards — Weaver's superhero team — appearing on a stupid morning talk show) to horror (an entirely new Endbringer — city-killing abomanation — attacking).
Season 3 finale of Noob has several in a row. Sparadrap's pets, the thing he cares the most about in the game get killed by enemy players, obviously shattering him. His usually selfish teammate Gaea calls the perpetrators "monsters"... because she was intending to steal said pets and sell them for lots of money (which suits her much better). Other usually selfish teammate Omega Zell reacts along the lines of "You're really a horrible person, Gaea". Then Sparadrap starts going berserk in radical contrast to his usually foolish attitude. But the enemy players are much stronger than them and quickly get the upper hand back. Then The Cavalry arrives just in the nick of time! Next thing we see is a clamed down Sparadrap, still mourning his pets while his brother cheers him up.
Near the end of The Cartoon Man, a tense, serious scene abruptly gives way to a wacky, slapstick cartoon battle scene. Then, when a non-cartoonified character is seriously injured, the scene suddenly becomes even more serious than before.
Invoked by this (expletive-filled,note although censored ranting) entry of Jacks Films' "Your Grammar Sucks" series. It inexplicably ends with the words "I like that", and Jack reads it accordingly.
Rachel Bloom's comedy music video I Steal Pets seems nonsensical and frivolous until the protagonist reveals midway through that both her parents are dead, providing the most likely explanation for her bizarre reaction to getting bullied at school.
"ok this is issue 10 of richie rich and jackie jokers and the previous 9 issues had covers that were typical money-based visual gags, and then suddenly this motherfucker comes along and not only are richie and jackie going to fucking die but so are billions in a forthcoming atomic war??? jesus fuck that is some tonal whiplash"