From Warhammer 40,000, the reaction of anyone, opposing player or in-universe soldier, to seeing a Chaos Champion being possessed by a Greater Daemon. Described in a few places in the background. One text box sidebar in either the Chaos Marines or Chaos Daemons Codex describes an Eldar autarch's reaction to seeing a Slaaneshi Champion being possessed by a Greater Daemon; his initial reaction is to resign himself to die fighting, but then its magic takes over and he stands there staring at it as it eviscerates him.
Also, when the Imperium tacticians were trying to figure out how the Tyranid fleet was appearing at random locations far too quickly, and then realized that the tyranid fleet was not coming at the galaxy from the edge, but from below the galaxy. You can see one single fleet's attack movements here (remember that this is one fleet, the Tyranids likely have thousands).
Any time a particularly powerful unit does anything, really. Especially if it's something like Drazhar and a band of Incubi declaring a charge.
The Eldar, who are an entire race of smug (but very competent) snakes, get one just about whenever they realize that the "Monkeys" or the barbarians are going to go down hard or win. Notable was at the Siege of Orar's Sepulchre, when they deployed the Avatar of Khaine, a shard of the soul of their god, and the most powerful fighter in a craftworld...then Marneus Calgar beat it to death.To be honest, it was written by Matt Ward, who is terrible at writing good fluff) They retreated.
When you manage to kill a Bretonnian Grail Knight... and remember your opponent has prayed and makes his ward save..
Chess: That moment you realize that you're well and truly screwed.
Exalted: It's been made explicit that this is going to be the general Yozi reaction to Heresy charms. When an Infernal creates "Swallowing the Scorpion" and snaps the tethers linking his shard to Lillun, the Phylactery-Womb will scream in pain - and it's around that point that Malfeas, the Demon City, literallyshits bricks.
Go - Famously happened in the Ear Reddening Game; the observing doctor didn't know a thing about the game, but he definitely noticed the reaction of the white player when the opposing player made a brilliant move.
Monopoly: When an opponent is building houses or hotels (especially in the light bluenote Cheapest group to improve in regard to its rents, orangenote Second cheapest group and is frequently landed on because of jail, rednote The third red property is frequently landed on and bluenote highest rents in the game groups). Bonus points if it's the third housenote The third house drastically increases the rent of a property in comparison to other upgrades in the property.
May also happen if a player owns all four railroads. The utilities pale in comparison to the railroads, though.
Anyone who's played Arkham Horror knows this one: 8 players + 1 open gate + monster surge = oh crap. (For those who don't know, this means that 8 monsters are all about to appear in the same space.)