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Gohan had to punch Goku in the cheek in an attempt to get Goku to snap out of being frozen in terror after Vegeta went Super Saiyan 3 in Goku's Wish: The Return of Broly. Subverted in that it doesn't quite work.
In Reflections, Princess Luna does this to a grief-stricken Princess Celestia.
In Relationships Series, Nanoha gets one from Vita as she becomes hysterical about the prospect of Yuuno dying on a mission, and Vita reminds her that Yuuno is determined to get back safely.
In Fail To The King, Almaz is subjected to this trope twice: Once in Chapter one to snap him out of his three-day long Heroic BSOD, and again in a later chapter to prevent another from surfacing in the middle of an invasion. Both played largely for laughs, naturally - this is a Disgaea fic after all.
In The Second Try, Asuka delivers this trope almost word for word to Shinji in chapter 2, thankfully without the slapping. It is possible that had this occurred before the Third Impact, she would have slapped him, but she was probably afraid to (given what happened during instrumentality, and the fact he's the only other person left on earth).
At one point during his side story of the Pony POV Series, Shining Armor has to get headbutted by Captain Baseplate when he freaks out over the existence of the SAS (Equestrian Black Ops).
A Future of Friendship, A History of Hate: During the climax of Episode 5, Scootaloo, who passed the Despair Event Horizon when she realized that her wish to be an adult not only blew up in her face but also enabled Miserain to empower a woebeghoul, which is now destroying Ponyville, states it would be better if she had never been born. Rainbow Dash responds by smacking her, telling her to never belittle herself like that again, and giving a Rousing Speech that pulls Scootaloo back to normal and enables her to help Dash destroy the woebeghoul.
In Post Nuptials, Twilight floors Shining Armor with a punch to the face when, in the midst of a Heroic BSOD, he considers canceling the wedding.
Subverted in Wedding Bell Blues: Shining Armor is screaming continuously after learning that he's being forced to marry Queen Chrysalis. Another pony slaps him, whereupon Shining says "Ow" - and then carries on screaming.
In White Devil of the Moon, Nanoha's in the middle of a Heroic BSOD after finally confirming that she is the resurrected Princess Serenity, that her past life was just one big colossal failure, that the Senshi's sacrifices were in vain becauseSerenity killed herself in grief, that this is could have been avoided if Queen Serenity did better in teaching Princess Serenity and that all of these memories are driving her to the point where she isn't sure where Serenity ends and she starts. Fate solves this problem by making her feelings known and kissing her.
In Mega Man: Defender of the Human Race, Roll does this to Mega in episode 11, slapping him and lecturing him back to his senses—though it takes more than that to fully convince him.
Miller: She is Piggy, Miller's Daughter. And I am her father, Miller, Piggy's Father, your Majesty. No, wait I am not the majesty, You're the majesty, and this is my daughter, Piggy, and this is— (Loyal Royal Advisor backhands him) Miller: Thanks, I needed that.
And it's done again at the end of the Intermission.
Gonzo: Intermission!? Wait a minute!! Why wasn't I told about this?! This is an outrage!! OOF!! Thanks, I needed that. Rizzo: Don't mention it.
On an episode of the short lived MTV series Super Adventure Team, Chief Engineer Head literally slapped Dr. Criswell blind trying to get him to snap out of his funk.
In the "Give Me Your Money" number from Avenue Q, Nicky refuses to give up his money by ranting "I can't! I need it! I'm homeless! I can't! I need it! I'm homeless! I can't...." until Princeton slaps him. Nicky calmly hands over the money then.
Hamish: Well, don't just stand their applauding, do something!
Dougal: All right!
SLAP! SLAP! SLAP!
Dougal: Now, control yourself. Or next time it's the face.
Fairly common among the Imperial Guard of Warhammer 40,000. When they're not being executed for cowardice, that is.
Orks have a similar rule where morale tests can be passed by the boss dealing a few good thwacks to his underlings.
The Player's Handbook 3 for Dungeons & Dragons 4th Edition includes Skill Powers, such as "Snap Out of it", which is pretty much this trope.
A smack across the face brings your ally back to his or her senses.
General George S. Patton once famously slapped a young soldier who seemed to be suffering from shell shock (it turned out later he had malaria). It almost ended his career, proving that this isn't a good idea to try for real.
As a side note, the soldier Patton slapped later said, "He was probably pretty well worn out himself." Combat can do strange things to people. Patton's popularity saved him.
It doesn't work if it is "shell shock" (PTSD) either. Anyone who's been in the Armed Forces can tell you that, combat experience or not. A person that far gone has usually spent themselves keeping a hold for as long as they have.
This was parodied during the extensive Patton movie pastiche in The Simpsons episode "Bart the General". Bart does it, but Grampa scolds him:
Grampa: You can push them out of a plane, you can march them off a cliff, you can send them off to die on some God-forsaken rock, but for some reason you can't slap them.
MythBusters tested this trope on their December 22nd 2010 episode. Their result? Confirmed. While not up to control, all three testers did better in practical tests when frazzled and then slapped than when unslapped.
At least when the subject isn't suffering from PTSD, as above.
Depends, if the subject is already in a situation which can trigger a psychosis episode, it can actually snap them out of it. Largely due to the endorphins that get released from the sudden shock.
This is because being slapped activates the flight-or-fight response. PTSD is this response having caused too much stress leading to a mental breakdown, so activating it could actually make it worse.
Even earlier than this episode, they examined this trope in terms of a "sobering up" technique after getting drunk and deemed it plausible. Cue Adam getting slapped by Jamie (Cue to 1:47).
In some sects of Zen Buddhism, a monk walks the room during meditation sessions with a light, flat stick. When a meditating monk loses his focus because he starts to get tired, he will lean forward to get a light slap between the shoulder blades with the stick, which triggers the reaction to make him fully awake again.