Scary Surprise Party

A character getting kidnapped, told that something nasty will happen unless they go somewhere or someone (the character or someone else) believing they are in mortal peril, but it turns out to be a surprise party. An elaborate Practical Joke. Often a subtrope of Kidnapped by an Ally.

Examples of Scary Surprise Party include:

Advertising

A commercial for the Japanese Hibachi restaurant, Benihana, is done in anime style, and gives the appearance the hero's girlfriend has been kidnapped. But the point was simply to get him to Benihana so they could celebrate his birthday.

Anime and Manga

Inversion in Umineko no Naku Koro ni: HAPPY HALLOWEEN FOR MARIA. Rosa and the others find this message written on the door of the chapel. Oh, so it's a Halloween party for little Maria? How nice! Let's just go on in and - OH DEAR GOD!

Film

The film The Game is pretty much the canonical example.
At the end of Miss Congeniality, Grace is called by Victor to examine a bomb of some sort planted at the Miss United States after-reception. Turns out that the girls just wanted to thank her for saving them all and Victor knew the only way to get her to come was "if there was the chance to shoot someone".
In Mamma Mia!, Sophie's fiance is "kidnapped" by his friends for his bachelor party.
At the beginning of Get Shorty, the mob boss Momo goes to what he thinks is a dangerous sit-down with rival gangsters. It turns out it's a surprise birthday party. He promptly keels over dead.

Literature

A short variant occurs in Curse of Chalion: On a leisurely ride in the countryside, Cazaril and the princess he's working for are surprised by a band of robbers who turn out to be mostly young nobles meeting them for an unexpected picnic. The bit that scares Cazaril, a veteran of several wars, is how close he came to killing some of those idiots before he caught on.

Live-Action TV

Spoofed in a sketch by Conan O'Brien, where they showed "foreign Reality TV Shows". One of them was "Surprise Party or Coup?" where African dictators are led to a room, and they have to guess whether it's a surprise party or a coup d'etat. "Yes, it's a party... a murder party!"
Taking this to rather an extreme level, MacGyver gets taken at knife-point by a disguised Jack Dalton to a birthday party.
Subverted in 3rd Rock from the Sun, when a surprise birthday party is planned for Dick (at his own request—he's a weird character), and conflicts with a kidnapper's plans. (Actually, the kidnapper wants to cut his head open to find the transceiver that is really buried in Harry's head.) The Solomons are all tied up in the basement and learn what's going on, but guests keep arriving for the party and assume that the kidnappers are "catering", until being knocked out and tied up themselves.
Chuck's friends attempt to kidnap him as practice for his soon-to-be brother-in-law's bachelor party. Which they aren't invited to.
- They later do finagle invitations and "kidnap" Captain Awesome for the bachelor party.
Used a couple of times in Buffy the Vampire Slayer:
- Played with in Surprise. The Computer Science teacher, Jenny Calendar, whom the audience has just learned has a hidden agenda, intercepts Buffy on her way to a meeting with Giles, and tells Buffy he's asked her to take Buffy to meet him elsewhere. The audience is led to believe that Jenny is up to no good, but it turns out she's just taking Buffy to her surprise party.
- Buffy is railroaded under threat of fire-breathing demons to her surprise 19th birthday party.

Willow: Guess you won't be killing anything tonight, after all.
Buffy: Don't be so sure...

In All the Small Things, Esther and Michael see peculiar lights coming from inside the church as they drive past on their way home one night. They stop to investigate, wondering if it's being burgled, when suddenly the lights all come on, revealing that it's a surprise party to celebrate their wedding anniversary. Esther was in on it.
One episode of CSI has the male leads abducting one of The Lab Rats as he's leaving the building, to take him out for a birthday dinner. Unfortunately for the birthday boy, the "party" keeps going downhill even after The Reveal.
For Monk's birthday, Natalie decides that she will throw him a surprise party, no matter what. After her first few attempts fail because Monk figures out she's planning a party and refuses to go with her, in the middle of a case, she has someone follow them, gets the captain to fake a warning about them being followed and in danger, convinces Monk to hide in a dumpster, getting them wheeled into the surprise party. Monk (and probably the viewers) never saw it coming for once.
Dexter's buddies bag him up and toss him in the trunk of a car in order to escort him to his bachelor party. Problem is, Dex was already anticipating a rival killer's plot against him, and so he escapes his bonds and deals a fierce punch to Masuka's eye when his friend opens the trunk. Whoops.
The staff at ER frequently used made-up medical emergencies as a way to get the victim into the room where the surprise party was being held. And they could never say no and take the chance of it being a real emergency.

Video Games

The computer game Draculas Secret ends like this.
In the third Phoenix Wright: Ace Attorney game's fifth case, Butt Monkey Larry Butz has a crush on the defendant, Iris. However, he's so bad at writing love notes that his request to her to come see him in the middle of the night for a romantic excursion is taken instead as blackmail, especially since Larry believes Iris has a secret love for him. Iris took this "secret" to mean something different and much more plot-relevant, however...
Double subverted in Irisu Syndrome. Irisu plans this sort of party under the guise of this to murder two university students, and depending on which ending you get, she either carries the murders out or plays along with the planned party.
- Or go along with the party and commit one or two murders afterward. It's a bit vague there.
In RPG Maker-based game Ao Oni, this trope is cruelly subverted. The hero, Hiroshi, is suddenly attacked by the monster that has been chasing him and murdering all his friends, only for the Oni's head to pop off, revealing that it was a guy in a suit all along and that the whole ordeal was just a setup for a surprise birthday party. Pity that this turns out to be just a dream, and his friends really are dead.
In Bugs Bunnys Birthday Blowout for the NES, Bugs Bunny is attacked relentlessly by every member of the Looney Tunes cast, but finds out after the attacks that they were all just joking, and they set up his birthday party.
In The white chamber, the comedic ending which requires you to get six chalk marks (six good deeds) to obtain. as Sarah leaves the White Chamber, she returns to the ship via the vent duct only to see everyone shouting "Surprise!". It turns out it was her birthday and her surprise was being scared to death. Sarah and the crew make no mention whatsoever of the horrors she just encountered (or the fact that she was impaled at one point and electrocuted at another) and sit down to have cake. The ship is then unexpectedly blown apart by an asteroid, but instead of an explosion, a bubble appears that says "BANG".
Deconstructed in a sidequest in Star Wars: The Old Republic, where an extremely paranoid Imperial officer became convinced his men were plotting against him and had them all executed, despite the fact that they were simply planning a surprise party for his promotion. He then tries to convince you that the woods are full of invisible Kuubaz infiltrators.

Western Animation

The Sealab 2021 episode "Der Dieb". What annoyed Quinn was not only the abuse they put him through to get him to the party, but also the fact that it wasn't actually his birthday.
- "Broken hands, get it?! Broken hands!"
The Harvey Birdman "Deadomutt" two-parter where they framed Harvey for murder and let him spend five years in prison.
Featured in The Raccoons episode "Surprise Attack!", where Cyril Sneer mistakes plans for a surprise party as a plot against him.
Featured in The Simpsons. In an episode reminiscent of Eternal Sunshine of the Spotless Mind, Homer's journey to track down the secret of his erased memories lead to a surprise party on a party boat.
In The Grim Adventures of Billy & Mandy episode "Tastes Like Chicken", Billy finds his friends and family missing. Being the paranoid peabrain he is, he suspects they were all devoured by cannibals. Eventually, further subtle evidence seems to verify this. Towards the end, he's surrounded by cannibals, but they reveal themselves to be Billy's family, who prepared a tiki-styled surprise party for him. However, when he asks where his friend Irwin is, Mandy pulls his glasses from between her teeth, suggesting she DID eat him.
In Hey Arnold! Arnold and Gerald over hear two guys who they believe are attempting to take out Mr Green, but it was all a huge surprise party for him. Or is it....