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.
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 Demo Reel, Donnie has an amusing "ah fuck it's happened again" expression and groan when Egoraptor is in the bed next to him instead of Uncle Yo.
From AJCO: The ghost of Mercy, a bandit slaughtered by Curls some years prior, tries to hide the fact that he has an Oh Crap moment when Kaja shows up - Kaja being a powerful angel with the power to exorcise him from the body of the helpless girl he'd possessed without hurting her (therefore making his threats to jump off the tower meaningless). He eventually accepts his fate.
In the 6th episode of DesuDesBrigade'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.
Luigi, upon realizing that Mecha Sonic is about to Spirit BombDeath Ball the fucking hell out of their island Frieza-style, releases a single, utterly hilarious scream of sheer terror. It is FANTASTIC.
Nearly every Let's Play ofThe Witch's House on the internet causes the player to have moments like these, as traps are easy to fall into. The amount of time that the player has to realize how screwed they are varies from trap to trap. PewDiePie's playthrough is a good example, as is this one.
Mini Drivers, a Formula One cartoon, often exaggerates the various driving styles, quirks, etc. of the drivers in the series. So in this 2012 postseason video, two very crash-prone drivers (Pastor Maldonado and Romain Grosjean) start on the front row of a race, which immediately makes Mini-Girl (the starter) nervous. Once the field takes the green, every driver behind them Oh Craps when they realize the two are about to start a crash that, if the "achievement" is to be believed, will rival the alleged Mayan apocalypse.
Atop the Fourth Wall: During the last bit of the Silent Hill: Dying Inside review, the second Pyramid Head of all beings jerks back in fright upon seeing Linkara's new weapon.
In Worm, Tattletale gets one when she realizes that Scion is the one who will destroy the world.
Davison: No, no, the convention's not until... Oh my God.
Every microwave show at one point or another has one of these episodes
Is It A Good Idea To Microwave This most famously has the airbag episode and to a lesser extent the second lava lamp episode. In the first one, Jory was literally SECONDS from being killed. The reaction afterwards is priceless. However, that wasn't their first "oh crap" moment, in season 6 when trying to put out a fire from a singing pen, the battery explodes and Jory is surprised he doesn't lose his hand. The exploding rubbing alcohol episode may also qualify due to their reaction of genuine fear. The dynamite, propane tank, gasoline, and other similar episodes are NOT this because they KNOW of the danger potential.
In a sense their microwaving a microwave episode qualifies since that was the first time they broke a microwave that wasn't at the end of a season, and so they were unprepared for it. The latter half of the season isn't even filmed until it's almost time to air the microwave a microwave episode.
dOvetastic Microwave Theater rarely has these since dOvetastic takes TONS of precautions, the closest example to this trope being the propane tank, which he actually gets to explode on its own (Jon and Jory used gasoline and fireworks to get it to blow up) and possibly the dry ice bomb specials. The videos that DO qualify for this, however, are not his microwave videos but things like where he saws into a propane tank or any high voltage cooking episode with anything explosive involved. The deck varnish and CO 2 cartridge special applies too since the cartridges launch over 100 feet.
What Happens When You Microwave This? has a few notable episodes, but the only true Oh Crap moment happened with a lava lamp since they weren't expecting it to blow up like that. The bag of popcorn for 35 minutes is a bit of an ironic result since they thought it broke their microwave after burning for so long. The cameraman thought it was hilarious that after doing so many dangerous things, it may have been popcorn that broke the microwave.
Microwave Me has had very few incidents like this since the creator always seems prepared for the result, even treating a butane lighter with a bit of healthy respect, but the one that sticks out is the champagne bottle. Apparently he thought Brainiac faked their explosion (due to some highly unlikely experiments like the Newton's Cradle episode and admitted fakery of their alkali metals in a bathtub experiment) and that the bottle would only take the door off the microwave. What actually happenned was an explosion very similar to the Brainiac episode with the door being launched quite a distance (in three pieces to boot), the microwave being ripped almost in half, the turndial ripped nearly off of the microwave, and the explosion was supposedly as loud as a large firework mortar going off, like one would see at a professional display.
Smaller microwave shows rarely get this, but you do see this occasionally when they only have one microwave on hand and their experiment unexpectedly wipes it out. Most notably Jzizzle77 who microwaved high profile things like CO 2 cartridges, aerosol cans, road flares, model rocket engines, and even a mini-printer. His experiment microwave broke after the mini-printer.
A hilarious one happened in the Let's Play Grand Theft Auto V episode "Path To Insanity" where Ryan's killstreak is ended by Ray riding in a tank. Ryan is minding his own business, killing people as he attempts to raise his insanity meter when Ray casually strolls up in a tank on the street.. Ryan's reaction:
Ryan:OH, SHIT!Ray's in a tank!
In RWBY, Ruby has this reaction when Ozpin reveals just how random Beacon's partnering system is. Bear in mind that she's at a military school.
"The next person you make eye contact with will be your partner for the next four years."
In Thalia's Musings, this is Thalia and Apollo's reaction when Eros (falsely) tells them that he's dosed them with a love potion.
The Cockroach Metamorphosis has Thoth wrestling with a Nyarlathotep in bear form (in free-fall!) only for the both of them to come to the realization that they are currently fighting inside an even bigger god, and as they try to sneak out without waking it up, they turn around to see an ominous pair of eyes staring right at them. Even The Crawling Chaos himself flips his shit in panic.