Tex in Red vs. Blue gets this moment when Donut lobs a plasma grenade over an incredible distance, square into the cockpit of the tank she was driving.
Everyone is one the receiving end of this at least once, although their reaction is less "oh crap" and more "son of a bitch!"
In Recreation Episode 12, Donut is cleaning up Blue Base when he's approached by what he thinks is a new Blue soldier (it's actually the previous season's Big Bad, Meta.) Donut, being Donut, tries to strike up a friendly conversation with it, evading its attempts to kill him by near misses. Simmons then arrives, Donut tells him about the new soldier, which leads to this:
Simmons: "What? They sent another team member? Why would they do that? That doesn’t make any sense- (sees that it's Meta)OH, FUCK!(starts to run away) Welcometotheneighborhoodseeyoulater!
Revelations Episode 10 is filled with a ton of them from Grif, Sarge, Simmons and Tucker while they're all in the middle of being on the receiving end of a brutal Curb-Stomp Battle at the hands of Tex.
When The Meta attaches the memory unit containing the recently captured Tex to his armor.
The Meta is the living personification of this trope. He even has his own Theme music which makes the Imperial March, Jaws theme and Psycho Strings look tame in comparison.
And now try this. If you thought the last one was pants-wettingly terrifying, then this one makes the previous link sound like a frolic in a happy meadow.
The Oh Crap factor doubles down in subsequent seasons as we see the process that took him from "just" an exceptional supersoldier to an unstoppable force of evil.
The Demented Cartoon Movie: An underwater diver recites the trope by name upon realizing that the "Bubble Fish" he has been tracking is actually Zeeky H. Bomb.
Happy Tree Friends is a total definition of this trope. Nearly every episode has it's Oh Crap moments (save for just a few). In the fanimation "All Work and No Belay", before Lumpy falls and rolls into a big snowball, he notices what will happen and holds up a sign saying "HELP?".
In the first episode of DSBT Insani T, Koden and Asia have a moment when they realize Lisa and Sand Snake are going to crash Koden's party.
Happens to Koden again in 'The Camping Webisode' when he finds out Killer Monster's name.
Reverse Jurassic Park: Used word-by-word by Mr. Robustus in the fivequel when he realizes that the giant aquarium he is standing in front of just cracked
