Mood Whiplash/Web Original

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.
- Tourist: ...Okay!
This Cyanide&Happiness short does this over the course of 30 Seconds
- - This one as well. And then it lashes back.
The "Child Molester" clip from AMV Hell 4. It's an hour of silly jokes and you've long come to expect Dead Baby 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:

Anyone with half a brain
Could spend their whole life howling in pain
Cause the dark is everywhere
And Penny doesn't seem to care
That soon the dark in me is all that will remain...

- Joss Whedon: your one-stop shop for Mood Whiplash.
Survival of the Fittest, being an RP board, can have this happen with someone barging in during a dramatic moment.
- 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.
Tiberium Wars. Annual Black Hand Taco Fiesta.
Monstro_draw brings us "Cat Rackham and the Comforts of Life", a lighthearted, NSF56K story about a cat who wants some coffee and AHH! AHHHH!
A Very Potter Musical is mostly a light-hearted parody of Harry Potter but there is the occasional moment where-based on it following the series's plot of all things-you will get serious whiplash.
In Human Centipede the Musical, Heiter interrupts Steve’s cheerful music number by drugging him and dragging him offstage.
Colour My Dreams starts you off in a bright, sunny area with stuff to do. Then you pick up the phone and...things change.
Mr. Literal by Egoraptor. Whips so hard you'd risk breaking your neck.
Desert Bus for Hope 2010 saw a very sad story told about a young girl being treated for cancer. The whole cast was beside themselves with emotion and sorrow. Then, in the silence...

Graham: So, who wants to sing The Internet is for Porn?

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 Just Bugs Me: ranging from an ardent discussions of cultural perceptions to vehement attacks against political beliefs to injokes to a discussion about how awesome boobs are.
This picture: [1]
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.
You're Entering Puberty Charlie Brown starts off as a funny Lampshade Hanging Lampshade on the Peanuts universe then goes into Tear Jerker mode with snoopy dying all alone.
Entry #46 of Marble Hornets contains a) two of the funniest lines in the series and b) the most effective Jump Scare yet.
Brought up in cautionary fashion at the opening to Chapter 4 of the My Little Pony Friendship Is Magic fanfic The Monster Mash; given that Chapter 3 has Rainbow Dash end up as a Kaiju and chapter 4 is about Pinkie Pie's ghost interacting with her grieving friends, it's hard to call it unjustified.

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 third year of The Questport Chronicles begins with the heroes still celebrating last year's victory over the Big Bad. And then, out of nowhere, a sorcerer destroys Questport. The story picks up again with the few survivors searching for a new home.
Don't Hug me I'm Scared seems to start off as an Affectionate Parody of various puppet shows. Then the puppets finally start thinking "creatively"....
This video takes the original Downer Ending of Left 4 Dead's "The Sacrifice" campaign and, in the last seconds, makes it hilarious.
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 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.