Do Not Spoil This Ending

The Twist Endings of many shows and movies have become totally open secrets. Some productions have tried to avert this, admonishing audiences not to reveal the shocking twist or revelation that ends the show.

This is effectively secrecy on the honor system, and it might not last too long.

Comedy

 * Dave Gorman on his Googlewhack Adventure makes a big point of asking his audience not to spoil the ending.

Comic Books

 * There are a few covers on Super Dickery with a big box on the front of the comic saying something along the lines of "Don't you dare reveal the shocking identity of the most deadly villain ever!"
 * In Knights of the Dinner Table, anyone who plays the infamous module The Temple of Horrendous Doom has to sign a Non-Disclosure Agreement about its contents.
 * In this case, its to maintain the mystique of a dungeon that has a 100 percent kill rate
 * Peter David has asked this several times when bringing out a plottwist book.

Film

 * The 1956 movie version of The Bad Seed ended with such a warning. (It didn't deter the September 17, 1956 issue of Life from stating exactly how the movie's ending differed from that of the play.)
 * At the end of Witness for The Prosecution there's a disclaimer that reads "The management of this theatre suggests that for the greater entertainment of your friends who have not yet seen the picture, you will not divulge the secret of the ending of Witness for the Prosecution." Also, the trailers and promotional materials had a similar theme.
 * Initially, the promotions for Psycho had a "no late admissions" policy, to keep late arrivals from wondering where Janet Leigh was. Also, Alfred Hitchcock himself addressed leaving audiences over a loudspeaker by saying "Don't give away the ending - it's the only one we have."
 * Hitchcock often asked this of his audiences. With Psycho, he even went so far as to purchase all the copies of the book he could out of his own pocket to try and keep the twist a secret.
 * The 1956 French suspense film Les diaboliques (Diabolique in English-speaking countries).
 * Like Psycho, Les diaboliques had a "no late admissions policy" in its original run.
 * Moviebob on his Zombieland review insisted you to punch anyone trying to reveal something about the movie.
 * Then one of the guys who writes Cyanide and Happiness posted it on Twitter. Angrish resulted.
 * The 1926 film The Bat opens with an intertitle reading "Can you keep a secret? Don't reveal the identity of The Bat. Future audiences will fully enjoy this mystery play if left to find out for themselves."
 * Those who had seen the movie Fight Club, often quoted the in-movie rule "The first rule of Fight Club is You do not talk about Fight Club!" in order to not spoil the Big Reveal.
 * Fridge Brilliance: One of the two rules stating "You do not talk about Fight Club!" refers to the fight club proper, the other one to the movie?!
 * First rule: you do not talk about Fight Club. Second rule: you do not talk about Fight Club. Delivery with more emphasis = italics, italics denotes the title of a work. That IS brilliant.
 * Much of the marketing for the 2010 film Catfish centered around this trope, although the subsequent mediocrity of the film's secrets and questionable honesty of the filmmakers has not engendered much good will.
 * Critics at pre-screenings of The Crying Game were asked not to reveal the film's twist in their reviews. Surprisingly, this worked. The All There Is to Know About "The Crying Game" factor only kicked in after the film had debuted to the general public.
 * Manos the Hands of Fate, of all films, did this, with its poster saying "We dare you to guess the ending! - ...AND ASK YOU NOT TO DIVULGE IT!".
 * The trailer for Meet the Feebles ends with "And please, don't tell your friends how it comes out in the end." This hasn't stopped it from becoming a bit of a chore to find a review of the film that doesn't completely spoil the ending.

Live Action TV

 * Parodied on MST3K: while watching The Wild, Wild World of Batwoman, when Ratfink is unmasked, Tom Servo quips, "Please do not reveal the secret to The Wild, Wild World of Batwoman."
 * Same with Red Zone Cuba

MMORPGs

 * The online game Kingdom of Loathing included an obscure hidden puzzle whose solution included the message "do not spoil this", later revealed to have been an experiment by the game's authors to see how long it would take before the solution was revealed. Unfortunately, a group of people discovered the puzzle and started a collaborative project on the game's official forums to figure it out and share the solution, leading to some uncomfortable silence once they solved it.
 * In Billy vs. SNAKEMAN, the penultimate page of The Impossible Mission tells players that have gotten that far "One last page exists. The game's creator has asked that its contents be kept secret." The last page is possibly the most widely viewed part of the game that is not in its Wiki Guide.

Music

 * Weird Al's song "Nature Trail to Hell," which is basically an ad/trailer for the movie, admonishes "Please don't reveal the secret ending to your friends."

Tabletop Games

 * Paranoia distinguishes between rules ("don't read this section under penalty of termination, but if you do anyway, then don't tell anyone") and pre-written missions ("seriously, don't read this section, you'll spoil your fun"). Occasionally, the GM is reminded to watch out for this: "the password is 12312312-A, but if a PC just knows that, then they obviously read the adventure and should be terminated immediately".

Theatre

 * The ending of Agatha Christie's The Mousetrap has been an open secret for over half a century--part of the contract stipulates that the play cannot be adapted to film until after its West End production closes, and each performance ends with the cast ceremonially swearing the audience to secrecy.
 * The book it was adapted from, Three Blind Mice, was actually banned from publication while The Mousetrap was on the air (or at least, it says so in the book's preface).
 * Banned in the U.K.; not in the U.S.. Those spoilersports at That Other Wiki either are not sworn in or broke their oath, though, so avoid their article if you haven't read Three Blind Mice or seen The Mousetrap.
 * At the end of Ghost Stories, an announcer kindly asks that you not reveal the ending.

Video Games

 * Shift 3 asks you to reveal that the hidden character is someone completely different from the truth.
 * Both Shift 2 and Shift 3 ask you to tell everyone in your reviews that "The cc is a lie"  and how you hate that the secret character is Mel Gibson , respectively.
 * Also, Trilby: The Art of Theft asks you to tell other players that beating every mission in a row using a suit that increases your visibility unlocks something.
 * The creator of the adventure game "Diamonds In The Rough" asks the player not to reveal the ending.
 * The German RPG-Adventure Die Attentäter has a surprise twist ending, that at the time of publication asked for secrecy by the players. Even though another adventure, over a decade later, re-uses the secret that, you still better watch what you're saying at German RPG-cons so as not to upset players.
 * The creator of the Touhou Project has asked the fanbase not to reveal the endings of the games. Most likely so that those who do make it to the end and see an ending can feel a real sense of accomplishment. Given the genre, seeing the Good ending of a game is a pretty big accomplishment indeed. For the most part, the fanbase complies, to the point of the Wiki not revealing even the synopsis of any endings, and when a character only shows up in a game's ending, it's spoiler-tagged on the character's appearances list.
 * Applies to the near-endgame easter egg of Torins Passage. Creator Al Lowe kindly asks the player "But whatever you do, don't tell anyone how you got this message. Instead, just taunt them with: Why, I got a personal message from Al Lowe at the end - didn't you?"
 * Dwarf Fortress fans still refer to now-widely-publicized features in Fan Nickname only ("blue stuff", "clowns", "carnival", "cotton candy.") Some take this more seriously than others, but it's a firm convention even among those who don't.
 * Siter Skain, the makers of The Tale of Alltynex, have made this request of their fans as well. However, this is extremely frustrating for English-speaking fans of their games, since no translations, official or otherwise, have been made. A couple of bilingual fans have revealed the endings here and there, but good luck hunting those revelations down.

Western Animation

 * An episode of The Critic ended with Jay asking viewers not to reveal the twist ending of a plotline based on The Crying Game.