Oh Crap/Web Original

Dr. Horrible's Sing-Along Blog: When his freeze ray powers down, Dr. Horrible has just enough time to note "That's not a good sound..." before his face gets introduced to the now-free Captain Hammer's fist. Again.
Survival of the Fittest has Cody Jenson do this at the end of V1. He finishes off Sidney Crosby, who attacked him to save Adam Dodd, and then turns around just in time to see a very angry Adam Dodd attacking him with a katana. A quick 'Oh crap!' moment, considering that he gets stabbed and impaled almost immediately.
- Also happens to Harry Tsai in V3, when he sees Andrea Vanlandingham's detached collar, whom he killed earlier in the game, moments before Denise Dupuis wreaks her vengeance upon him.
During the That Guy With The Glasses One Year Anniversary Big Team Brawl, Ma-Ti manages to get his hands on The Angry Video Game Nerd's Super Scope. The Nostalgia Critic has just enough time to yell out "LOOK OUT, MA-TI'S PACKING!" , to which everyone adopts an Oh Crap look, before Ma-Ti opens fire, yelling "HEART!" at the top of his lungs.
- In his Top 11 F* ck Ups special, The Nostalgia Critic decides to hunt down Douchy McNitpick. Douchy is confident that there's no way the NC will find him, until he hears a window in his house breaking.
- In Suburban Knights, Team Awesome defeated Chuck Jaffers and his gang! But who's that across the street...? Oh Crap.
In the 6th episode of Desu Des Brigade's Pocky Spot, Arkada gets precisely this after hearing the Professor Otaku has to a little tidbit of info dropped.
During the RP of The Marios, this is the trio's general reaction when the creator of the bad LP in question, LuwiigiMaster, disses Retsupurae during his LP. Diabetus later comments that he should pre-screen LPs before bringing the gang over to riff them. If one reads the comments, the guy apparently was trying to purposely make a So Bad It's Good video just to get RPed.
- Made even more hilarious due to their depiction of said LPer as part of The Mafia.
The reaction of people in Mighty God King's version of Marvel's Civil War storyline when Thor shows up.
In The Demented Cartoon Movie, characters often say this if they have time to realize things are about to go disastrously wrong. And things go wrong (read: explode) a lot.
Super Mario Bros Z loves this trope, and all the characters get a moment of this at one point or another. The best ones probably come from the Axem Rangers Z in Episode 6 when Mecha Sonic shows up and starts wrecking their shit.
- Luigi, upon realizing that Mecha Sonic is about to ~~Spirit Bomb~~ Death Ball the fucking hell out of their island Frieza-style, releases a single, utterly hilarious scream of sheer terror. It is FANTASTIC.
Interactive Fanfic You Awaken in Razor Hill is loaded with these.
- At the very end, when our long-suffering protagonist has finally reached the Big Bad, only to discover that it's what has been guiding him through every last acre of hell, this entire time.
- Scrape.
Probably the biggest example of this has to be Jay of Marble Hornets revealing he did not post Entry ######.
- Entry #26. "When did we get a camera?" Though it's not particularly apparent on-screen, you can tell Alex has just realized "okay, this is really, really bad".
The title character of My Name Is Zytherys only considers it a bit odd when a mysterious journal shows up on her doorstep. Slightly more odd when it begins to fill itself with her own handwriting. However, when her laptop begins to show visual tearing and starts scrolling on it's own...
A dog gets pwned...
In some Protectors of the Plot Continuum missions, Explosive Instrumentation has been known to do this.
Tobuscus' Let's Plays are littered with this type of commentary, most often phrased as some variant of, "I'm screwed I'm screwed I'm gonna die I'm gonna die I'm dead I'm dead GOD DANGIT!"
In Pay Me, Bug!, this is Morgan's response when he realizes that they're being gravlocked by a Battlecarrier.
In the NES Godzilla Creepypasta, this is the player's reaction every time Red demonstrates that The Fourth Wall Will Not Protect You.
Any time Meta's theme starts playing in Red vs. Blue.