Hilarity Ensues because a character must perform an act that is either against their nature or which they have vehemently and constantly protested against doing. Said action can also be just plain humiliating or all of the above—to all parties, perhaps. When this act is aided or witnessed, a character insists that either they must all never speak of this again, ever; or that a specific person who is guaranteed to ride them never hears of it. A common reply to comply with the sentiment is say something to the effect of "Speak about what again?".
This trope can be used to introduce a Noodle Incident, with one character reminding the other they promised never to speak of that again. Alternatively they may have just completed the action, make some passing reference to how unusual or interesting it was, and then pull this line.
Subversions can occur when the character(s) kept in the dark reveal themselves to have known all along or if the character who would have used the incident as ammunition, against all expectations, chooses not to do so. Sometimes that somebody will bring it up again and earning the reaction "I thought we agreed to never speak of it again" or something close to that.
It can be parodied by the person(s) who are told to never mention it again being genuinely unaware of what happened for whatever reason, leading them to ask something like "Speak about what again?", which is then taken by the person wanting the secret kept as a joke implying compliance with their wish.
Compare You Didn't See That, Never Say That Again, That Didn't Happen, and Big Lipped Alligator Moment.
It happens again later under very similar circumstances, only this time the kisser was a Negi duplicate gone out of control, and the victim (and person saying it) was Akira.
In the Yu-Gi-Oh! GX dub, Elemental Heroes Sparkman and Avian realize they were brainwashed into loving the Maiden in Love: "Guy pact — this didn't happen, okay?"
During the first Yu-Gi-Oh!, Kaiba sums up the Noah filler arc as a "complete waste of [his] time and effort, so let's move on and pretend that nonsense never happened."
In the Naruto anime filler episode where Shino laughs and displays his sense of humor, he informs the title character in no uncertain terms to not mention it to anyone.
In Haiyore! Nyarko-san, the Pretty Freeloaders are setting up their own rooms and Mahiro goes to see how they're coming along. He walks into Cuuko's room and sees it's a Stalker Shrine to Nyarko...and Cuuko herself is in the middle of humping a body pillow with Nyarko's picture on it. Mahiro silently backs out, closes the door, and says "I'll just pretend I didn't see that."
In episode 10 of Free!, the swim club is having lunch at Goro's place when his stack of swimsuit magazines gets knocked over. As he picks them up, he looks them over and realizes that Miho Amakata is his favorite model, Marin-chan. He starts going all, "Oh my god, I was your biggest fan!" Miho gets very angry (she is not proud of her past as a model) and orders him to drop the matter and never mention it again.
In Chaos War #3, while under the influence of Venus's magic song, Amadeus admits to Thor that he loves him. Thor says he loves him too, and then says not to speak of it ever again.
A panel featuring these exact words appears in volume 4 of Scott Pilgrim, after Scott discovers a drunken Knives and Kim making out.
Volume 6 had Scott and Knives making out, after coming to terms with the end of their relationship. It was horrible. For everyone. And that includesyou!
In Fables, Prince Charming and Beauty are working in an office during a heatwave. Nearly delirious, they briefly make out. They awkwardly agree never to bring it up again, especially not in the presence of her husband, Beast.
Subverted in the new Ms Marvel. Wolverine would very much like to not talk about the heroine taking him on her back when he was wounded, but the girl unfortunatelly already uploaded the story on the internet.
In the Cropped Scan Theater rip of Yami no Matsuei, the first filler chapter which had no relevance to the rest of the plot was ended by this exchange:
Tsuzuki: And all's well that ends with shoujo bubbles and the acceptance of mortality. Now let us never speak of this again. Matsushita-sensei: And they never did.
Thereafter, every filler or otherwise irrelevant chapter was ended by the narrator saying "...And they never did."
Yugi: Well, it's the end of the movie. All in all, it was kinda disappointing. Téa: I can't help but feel it was all just a cheap cash-in made to promote a bunch of cards. Grampa Moto: It's almost like this whole story was thrown together at the last minute. Pegasus: I agree. So, from now on, let's pretend it never happened.
Freeza from Dragon Ball Abridged has one of these after an awkward conversation with Zarbon after the latter mentions his girlfriend.
Freeza: Oh... and all this time I could've sworn you were— ...never mind. Zarbon: What? You thought I was single? Freeza: Well, no, I just... I thought you were into... you know, it really doesn't matter. Zarbon: Well it matters to me, because, frankly it sounds like you thought I was— Random Minion: Lord Freeza! Vegeta's broken out of the healing—TAAAAANK!(dies by ki blast) Freeza: Oh no, that minion died. Could you go fix that? We'll continue this conversation never.
Post Nuptials: When Fluttershy and Iron Will are walking through the Canterlot hedge maze during the wedding reception, they stumble across Octavia and Vinyl making out and quickly walk away.
Iron Will: Did you see...
Iron Will: Me neither.
In A Door Ajar Sirius hid Harry's invisibility cloak right before he was supposed to be leaving for a new school, which prompted the following exchange:
Harry: Don't make me give you another flea dip, mongrel. Sirius: We agreed to never speak of that again.
In A Marauder's Plan Harry derailed Sirius' attempt to tease him by threatening to tell Remus about "the time Healer Fay was over when we were swimming and you lost your…" and Sirius protested that "We swore we would never speak of that." Later on, after receiving a second-hand sex talk from a pensieved memory of Harry's father, he and Neville agreed to never mention it again.
Their glares communicated much to the other and while the majority of their exchange consisted of varied insults and death threats, the one message they both shared for the other was, ”Let us never speak of this again.”
Hard Reset: Twilight and Spike have this as a standing arrangement regarding her time as an Emo Teen.
Luna: You can't blame me for what happened. I don't even know where the cultists found that much jam living inside a volcano, much less how they kept it from going bad.
Celestia: You could have warned me about the ferrets, at the very least. And then to top it all off this 'epic artifact of immense power' turns out to be-
Luna: The book didn't mention that! I don't know what the Order was thinking! I don't even know how you'd do that to a spatula! Those poor, poor quiches. Their sacrifice shall never be forgotten.
Celestia: Oh, yes it will. Because we are never going to speak of this again. Ever. Especially not to Twilight. Do you understand?
In Lightning and Thunder Remus started to mention the time they were playing indoor hide-and-seek and hid in McGonagall's classroom, only for Sirius to protest "We swore we wouldn't bring that up again!"
Sushie: Does this make up for the volcano thing? Sonic: …as long as we never speak of it again!
In The Search for Life and Death Remus was teasing Sirius about an accidental encounter with love potion and Sirius replied "The Great Potion Blunder of fourth year was never to be brought up again!"
Taken to a very dark and emotional extreme in Uplifted: Last Days when Joachim Hoch, with the assistance of Heinz Heydrich, Hans-Gerd von Rundstedt, Claus von Stauffenberg and Admiral Zorah saves Ten children from Buchenwald Concentration Camp. He does not want this act to be exploited or used to prove he's changing in fears that people will assume that it was a token gesture..
In Family Bonds Harry was teasing Snape about having gotten lost in the forest on his own inherited property and Snape replied "I thought we had agreed not to mention that ever again!"
In The Cursed Ron, while enthusing about Harry's first-year flying skills, mentioned how close he and Neville were to being turned into a pancake and Neville replied "I thought we had agreed never to talk about it ever."
The Infinite Loops will sometimes land Loopers into situations that they want kept off the record for the rest of their very long lives. Hence, this trope is prevalent.
Within the PokeumansFan Verse, not talking about the spinoff titled Chosen which broke almost every single rule the group had has become a sort of group meme.
In Tempest of the Fae one of the wands Luna was trying turned a table into a talking puppet which ran away before anyone could change it back and Ollivander commented "I believe we should all agree not to speak of that again."
Films — Animation
Crane says this to Po in Kung Fu Panda after Crane helps him out of an uncomfortable position.
Diego: If anybody ever asks, there were fifty of them. And they were... rattlesnakes."
Films — Live-Action
The Starsky & Hutch film has this in spades. After Starsky (Ben Stiller) and Hutch (Owen Wilson) have to do some fairly embarrassing things to get information out of a con, Hutch starts to talk about it in the car ride back, and Starsky shuts him. "No! No! Let's not talk about it. In fact, let's never speak of this again." Of course, when they get back to the station, it's been recorded and is playing in the Captain's office with all of the other officers watching.
In the film version of The Lord of the Rings: The Two Towers Gimli says the above quote before having Aragorn use him in a Fastball Special (a reference to a scene in the first movie in which he objected to being thrown by Legolas). Aragorn promises not to say a word.
An oblique variant in Spaceballs: after watching the painful footage of being catapulted into the bulkhead of Spaceball One, Dark Helmet says "No, no. Go past this part. In fact never play this part again."
The Hunt for Red October. After Admiral Greer fakes the torpedoing and destruction of the Red October, he tells the torpedo control officer of the frigate he's on:
Admiral Greer: Now, Commander... that torpedo did NOT self-destruct. You heard it hit the hull, and I... was never here.
In the end of Zathura, the sister Lisa says this to her younger brothers, mainly because she thought the future version of one of them was hot before finding out who he was. They bring it up anyway.
Burt Gummer says this after surviving being swallowed by a Graboid in the third Tremors movie.
Burt: I would prefer... we keep this... to ourselves.
Earlier, Vincent Vega and Mia Wallace both agree not to tell anyone, especially Mia's husband Marsellus, that she almost died of a heroin overdose and Vincent had to inject adrenaline directly into her heart to keep her alive.
Lords and Ladies: No one should even mention the Stick and Bucket Dance. Not ever. At least, not once the Lancre Morris Men are done using it to kick elven ass.
Also in Witches Abroad, after the witches take part in it, nobody mentions the Thing With The Bulls ever, ever again. At least not around the men.
In Pride and Prejudice and Zombies the Bennet girls come across a zombie mother and infant and find themselves unable to kill them. Afterward they swear never to mention it again.
Spoken by an assassin named Whip to his driver in the C.J. Box novel Stone Cold when he's being driven away from the scene of a hit he just performed and the victim turns out to still be alive and sits up (whilst in the body bag) in the back seat, prompting Whip to load another few slugs into his head for good measure.
Heroes. Just so there are no misunderstandings, Nathan can fly:
Matt Parkman:(after riding Nathan Petrelli to Texas) We never talk about that again.
In Seinfeld, George dates a woman who looks like Jerry. George tries to come up with things that the relationship can be 'about' so as not to deal with the idea that he might harbour homosexual feelings for Jerry. The woman gets gum in her hair and cuts it out, making her hair short like Jerry's. George runs away screaming. Later, at the coffee shop, George and Jerry are talking about the incident and George says 'We must never ever speak of this'. Afterwards Jerry awkwardly asks George if he wants to go and see a movie. George refuses, instead saying that he's 'going to take a few days off', which Jerry agrees is probably for the best.
Ross and Richard are having a sleepover at Rachel and Monica's. At one point they bump into each other in the living room, wearing their girlfriends' bathrobes. After a few awkward seconds they attempt small talk, but then Richard cuts to the chase and says "Let's never speak of this again." Ross says "You got it." and they move on.
Also in "The One Where Joey Loses His Insurance", Joey thanked Chandler for helping him take a shower when he had a hernia, and Chandler freaked out and said, "Now, is that never talking about it again?!"
And there's the time that Monica is stung by a jellyfish. A friend who will pee on you is a friend indeed.
The episode "The Naked Now" features all sorts of weird events when the Enterprise crew falls under the influence of a virus that basically makes everyone more than a little drunk (including Data). One scene features Data meeting up with Tasha Yar in her quarters, where they end up having sex. After the virus has been purged and everyone is sober again, Tasha and Data are back at work on the bridge, where she tells him, "I'm only going to say this once: It never happened."
In "Skin of Evil", Tasha's funeral speech was supposed to include an apologetic "Data, it did happen." It's clear to see where it was supposed to be, as they left in Geordi's reaction shot.
And it's also alluded to in First Contact. Thus, there are some instances where it's treated as if the above scene wasn't cut.
"In A Mirror, Darkly" featured this darkly amusing exchange between Mirror Tucker and Mirror T'Pol:
Tucker: "I did you a favor once."
T'Pol: "We agreed never to discuss that."
Tucker: "And I haven't."
T'Pol: "I'd hardly call it a favor, you enjoyed yourself."
Tucker: "Several times as I recall. So, how many years is it 'til your pon farr comes around again?"
Star Trek: Voyager. Averted when B'Elanna Torres tries this in "Blood Fever" after a similar Mate or Die is barely averted. Tom Paris stops the turbolift and insists they do talk about it, as they're going to be stuck on the same ship for years.
In one episode of Scrubs, Dr. Cox invokes this trope after Dr. Kelso gives an all-too-graphic description of his and Mrs. Kelso's recent sexual encounter.
Buffy: How bored were you last year? Giles: I watched Passions with Spike. Let us never speak of it again.
Spike and Andrew have an exchange on their mutual love of onion blossoms while en route to a mission, ending with:
Spike: Yeah. (beat) Tell anyone we had this conversation, I'll bite you. Andrew: Right.
A vampirized Harmony meets Xander and decides to feed. They apparently regress to their grade school enemy stage (since both of them can fight better), and they're so ineffectual that they eventually just give up. They agree it never happened, and both leave.
Xander after telling Buffy he was once roped into working as a male stripper. "...and no power on this Earth will make me tell you the rest of that story."
Angel played it for laughs in the episode "The Girl in Question". The head of Wolfram and Hart's Rome branch used it several times.
CEO lady(spit) We will speak of it no more!
In an episode of Would I Lie to You?, Neil Morrisey, voice of Bob the Builder, had as one of his "facts" that he once had a builder working at his house who was in fact named Bob. David Mitchell said that if that was true, it was completely unremarkable, and the conversation probably went, "Do you know, I'm actually a builder called Bob." "Right. I do the voice of Bob the Builder." "Right." [short laugh all around] "Let us never speak of this."
The West Winglampshaded its frequent use of of the Walk and Talk by having Sam and Josh walking through the crowded hallways having a conversation as usual. At the end, they both came to a stop and asked each other where they were going. Turns out they were both following each other.
The Doctor also had one of his own when the head of Torchwood found out that the woman he'd claimed was Rose Tyler was actually Rose's mother.
The Doctor: Please, when Torchwood comes to write my complete history, don't tell people I traveled through time and space with her mother.
The Big Bang Theory: After returning from a research grant in Antarctica, Leonard, Sheldon, Howard, and Raj repeatedly talk about how they weren't going to talk about "the night the heat went out." Sheldon, being Sheldon, brought it up anyway. (They huddled together naked).
In Babylon 5, after learning the secret of the 'tentacles' Londo were using to cheat at cards, Lennier decides to swear an oath of silence on the matter.
Between the Shay siblings, notably, Carly's No Periods, Period in "iDate a Bad Boy" and Spencer's attempt at giving Carly The Talk in "iSpeed Date".
Sam and Freddie's agreement about their First Kiss. When Sam's drugged slip-of-tongue reveals it to Carly, Hilarity Ensues.
Used hilariously in Rome. Cleopatra wants to use The Baby Trap on Caesar, but knowing that Caesar has no sons, (and possibly needing a distraction from her withdrawal symptoms after Going Cold Turkey) she decides to increase her chances of pregnancy by sleeping with one of the legionnaires that Caesar sent to escort her to him. She first orders Vorenus to sleep with her, but Vorenus (despite being sorely tempted) refuses due to fidelity to his wife and Roman sexual attitudes to the idea of being "used" for sex. Vorenus then orders the other legionnaire, Pullo to "Report to Princess Cleopatra and do whatever she tells you to." Cue a prolonged and noisy bout of sex, much to the discomfort of Vorenus. When Pullo returns, they have the following conversation:
Pullo: Gods, that was something, let me tell you. Vorenus: I don't want to hear about it. If you're wise, you'll never speak of this again. Pullo: Why? I was only obeying orders. Bloody good orders, too.
When the youthful Octavian comes home after being kidnapped in Gaul, then rescued by Vorenus and Pullo, his rather shallow mother fuses over him. "Was it very horrible? ''(Octavian opens his mouth) Of course it was, we shan't even talk about it. Let's put it totally out of our minds."
Invoked almost verbatim by Cristina in the Grey's Anatomy episode "Complications of the Heart", after a surgery wherein both she and the more experienced Dr. Altman forgot how to perform a very basic procedure and ended up needing the nurse to bail them out.
Almost verbatim on The Peter Serafinowicz Show. Upon making a breakthrough deduction in a case, Sherlock Holmes becomes so sexually aroused that he forces himself on Watson. In post-coital reflection, Holmes says "We shall not speak of this again."
In White Collar episode "Power Play" Neal and Peter have to swap identities. This leads to Peter asking the whole office to go along with it.
Peter: Listen up, everybody. For the next hour, and only the next hour, you will call Mr Caffrey Special Agent Peter Burke. No questions, no laughing, just do it. And never speak of this again.
JAG: When Admiral Chegwidden receives a mysterious tape with no return address in his office, he has Petty Officer Tiner play it. It is a video of the Admiral's fiance dressed seductively holding the roses he sent her quoting Shakespeare and inviting him to a very special dinner. The Admiral cuts the tape off midway and tells Tiner to never speak of this again. Tiner wisely claimed nothing happened.
Dean invokes this in Supernatural when he has to talk Charlie through flirting with the male security guard with Sam listening in. Sam never does mention it again, but that doesn't keep him from laughing his head off all the way through it.
In HarmoKnight, Master Woodwin, after talking to you once you beat the Final Trial level, tells you to never talk about him losing his staff again. Because, as he says, that would press the princess's Berserk Button.
There's a Mental Phlebotinum Breakdown in Sasha's mind, and a horrible mutated monster comes up and proceeds to pound the living daylights out of Sasha in a highly embarrassing fashion. Raz saves Sasha's life, for which Sasha grants him his merit badge. This conversation then occurs:
Raz: So, is this the part where I get a speech and learn another lesson...? Sasha: No. Here is your merit badge. Let us never speak of this again.
From Dragon Age: Origins, Sten says this to Leliana when she sees him playing with a kitten and picking flowers. Also comes up after the threesome/foursome with Isabela in The Pearl.
Zevran: I don't mind never speaking of it again, as long as we could do it again.
In Cave Story, one of your fights with Balrog begins after you almost trick him into helping you with a feat that requires great strength. When you win, he yields and assists you (this is one of the hints dropped to you that he isn’t really such a bad fellow; he's just doing his job). He entreats you, however, not to tell anyone he did it.
Abe Lincoln Must Die! has Sam and Max take a date test to find the perfect match for each other. The result gives... Sam to Max and Max to Sam. Both agree never to speak about it again.
One of the categories for the third annual Neopies on Neopets is named after this trope.
Dawn of War II agrees with fans' opinion of the previous Soulstorm:
Trucy: I can still remember that moment... You brandished those bloomers on high, and shouted... "Objection!" Apollo: ...Here's what I want you to do, Trucy. Take that memory, gently lock it away deep in your heart, and never speak of it.
You may say, "But I don't HAVE any enemies!" Well, do you have FRIENDS? Are they on FIRE? Right NOW? Jarate them to a non-on fire state INSTANTLY! Being on fire is horrible! They'll never want to talk about your Jarate rescue ever again, but you'll have their secret undying gratitude!
On the Special Delivery mission, once the rocket lifts off and immediately crashes the Announcer might have this to say:
Announcer: Gentlemen, this never happened.
In the Mass Effect 3 Citadel DLC, it is possible for Female Shepard and Javik to end up in bed together after the party:
Javik: It turns out there is one thing primitives are good at. Shepard:Not a word.
In Saints Row: The Third you need to impersonate Cyrus Temple, the STAG commander, to rescue the kidnapped Shaundi, who is being held in a STAG carrier chock-full of enemies. When you get to the second-in-command, the Boss tries to make some chit-chat. If you've chosen the "Cockney" voice, she'll notice and call you out on your sudden British mannerisms. The Boss handwaves this saying "Oh, I must have picked that up when I was stationed in Britain" and she replies "You've never been stationed in Britain, commander". The Boss' answer: "I know. Let us never speak of this again". Astonishingly, this works.
In Saints Row IV, after being rescued from a Zin simulation, Kinzie warns the Boss to never speak of the fact she was forced to wear a poodle skirt.
While being interviewed for the Lawful Good afterlife, hero Roy Greenhilt stops the deva interviewing him before she can mention a Gender Bender incident within earshot of his father, who is standing not ten feet away.
During that very incident, Roy also insisted Belkar not know. Subverted in that Belkar knew it was Roy all along and was comfortable enough with his masculinity to hit on Roy purely to mess with him.
And during Azure City's New Year, Belkar gets drunk and kisses Vaarsuvius full on the lips. Afterwards, Belkar forgets it entirely and Vaarsuvius insists that, if the one witness to the kiss (Durkon) ever has to discuss it again, it will be referred to it as "the Event". And they never talk about it again.
When Vaarsuvius is rescued from the Para-Elemental Plane of Ranch Dressing, he/she mentions that there were some adventures before they got there.
Belkar: Maybe we'll see that as a bonus story in one of the books!
In another instance, Grace and Sarah attempted to prank Tedd by using a transformation device to make Sarah look exactly like Grace. Unfortunately, when Tedd encounters Sarah as Grace, he immediately kisses her. While there is no dialogue in the comic itself, the commentary states that they decided to never speak of it again.
Crystal: My head is killing me. Does anyone remember what happened after the tequila shots? Riff: Let's see. We're charred, we have twigs in our hair and we're covered in ducks. Bun-bun: I vote we all go home and never speak of this again.
Bar'd: Vas uses his change of gender to quell the rage of one of the more violent patrons by... stuffing her face into his new rack. It takes a bit for this to register with him, though.
In Questionable Content#447, Marten and Marten's mother, who is a professional dominatrix return to his apartment to find Pintsize wrapped up in bondage gear, which prompts this line to be spoken.
In Homestuck, Karkat takes this to the logical extreme by trolling John backwards, so that they'll never be able to talk about the first conversation (from Karkat's perspective), where Karkat accidentally reveals his hatecrush for John. Needless to say, it's hilarious.
Dave also does this early on in the strip, after royally screwing up his fetch modus and losing the beta in the process, despite there being no one around to have seen him.
In Housepets!, Peanut Butter and Grape happen to see Fido and Sabrina do... something, in this comic. Afterward, Grape tells Peanutnote who took a picture of the "something", though the viewer never gets to see it, "We never speak of this again."
Used by the characters after a revelation from Tailsteak.
A Running Gag throughout the Mario Party TV group's first playthrough of Mario Party 9 is agreeing that they'll finish this run, set the game aside, and act as if it doesn't exist.
Agent Supernumerary of the Protectors of the Plot Continuum is quite specific about the fact that when he says it's "unspeakable," that means he does not want Agent Suicide to keep talking about that little incident with a reality-warping typo and some very surprised goblins.
At the end of Don't Hug Me I'm Scared, when the characters' attempts to be creative result in some...stuff, the Notepad finally concludes "Now let's all agree to never be creative again."
In "Purple Party Pooper", Twilight and Rarity accidentally mention humans. Humans are apparently the Elephant in the Living Room, with some dire but unstated consequences for acknowledging them. Twilight and Rarity both panic briefly, then agree to just pretend that the entire conversation never happened.
In "The Cheesen One", Scootaloo has to fill the vacated village idiot position while Rainbow Dash is on vacation. Afterwards:
Rainbow Dash: Thanks for covering for me while I was gone, Scootaloo. Scootaloo: <Don't ever mention it. Ever.>
South Park: In the "Fantastic Easter Special" episode, Jewish Kyle and Jesus Christ are locked in a cell. Jesus can escape to stop the episode's villain, but only if Kyle kills him so he can resurrect outside the building. Being notably Jewish, Kyle is less than enthusiastic about the idea, but eventually agrees on the condition that, "Eric Cartman can never find out about this."
Marge also said it in regard to a relative who snapped and went on a shooting spree:
Marge: Well, Bart, your Uncle Arthur used to have a saying: "Shoot 'em all and let God sort 'em out". Unfortunately, one day he put his theory into practice. It took 75 Federal Marshalls to bring him down. Now let's never speak of him again.
This also happened after the episode where Principal Skinner was revealed to be really an imposter named Armin Tamzarian. They cleared up the problem by deciding they were happier with the fake Skinner, and the real one was run out of town. The local judge issues a decree that no-one will ever mention it again, under penalty of torture.
And, this beingThe Simpsons, they do mention it again — when Skinner is giving Lisa a hard time about naming a new cat of hers the same as her old cat, she calls him Mr. Tamzarian.
On the DVD Commentary for I, (Annoyed Grunt)-Bot, the episode with that Call Back, Al Jean indicates he inserted the joke himself as a Take That to that episode.
Another episode introduces Grandpa's brother Cyrus, with dialogue to this effect (paraphrasing):
Bart & Lisa: You never told us you had a brother, grandpa. Grandpa: Yes, and I'll never speak of him again.
The Powerpuff Girls had episode "Shotgun Wedding", in which the Professor tries to learn more about Fuzzy Lumpkins for the purpose of science, and after getting covered in mud, flowers, and a curtain is mistaken for a woman by Fuzzy and nearly forced to marry him. After the Professor is saved by the Powerpuff Girls, he writes on notepad that he was using "Lumpkins are, above all, stupid" and as a sidenote "never speak of this again"... and then eats it. He gets really mad when the Girls mention it about 10 seconds later.
Grim: We keep flying, we never head back, and we never talk about this again.
The 2008 BotCon script reading, "'Bee in the City", ends with all parties involved agreeing to never speak of the incident again.
In Hey Arnold!, whenever Phoebe ends up involved on something involving Helga's crush on Arnold, Helga always tells her this at the end.
Helga: This whole thing? It never happened. Phoebe: Forgetting!
Principal Wartz:(in a sombrero shaking maracas) Are you attempting to leave the school grounds without permission, Miss Pataki? Helga: Are you attempting to dance the flamenco, Principal Wartz? Principal Wartz:(beat) This moment never happened. Helga: Gotcha.
In Recess all the group agreed this when they see Spinelli having fun dancing Hula Hula with Finster, who was babysitting her in that episode.
In the Justice League episode "This Little Piggy" we get to see how far Batman will go to save Wonder Woman here.
In one crossover between Teenage Mutant Ninja Turtles and Usagi Yojimbo, Usagi says this when he and Leonardo put on black cloaks and infiltrate a fortress. While Usagi respects the Turtles, he doesn't really like ninjas and their "cowardly and dishonorable" ways.
In The Penguins of Madagascar, ("Jiggles" episode), after Kowalski's Blob Monster spat both Kowalski and Julien out, the two were covered in green goo and draped across each other. Julien's response to the situation is, "I will not talk of this if you don't."
Ikazuki: Well? What are you waiting for?! Ratso: Uh, well, this is kinda awkward. Ikazuki: Simpleton! Do as you are told! Finn: What he said. Ratso: Okay! But we are never gonna bring this up again. Chow: I'm not even here.
Danny Phantom: In "Reality Trip", Sam wonders how Danny convinced her parents to let her go on a cross-country trip with him and Tucker, and the explanation is thus:
Sam's Mother:(being overshadowed by Danny) You know I think the Fentons are wonderful, Sam should take a cross-contry trip with them. (Danny pops out of her, and overshadows Sam's father) Sam's Father: And I agree with you! Let's never speak of this again.
In Wakfu season 2 episode 5, "The Dragon-Pig", for not listening to Sadlygrove's warning, Evangelyne, Amalia and Yugo fall victims to a Baleful Polymorphturning them into cute, but defenseless piglets. Once they regain human form, Amalia insists that this incident should never be mentioned again.
In the All Just a Dream episode of Kim Possible, Kim saves villains Drakken and Shego from an exploding device. After Shego realizes how awkward it is, Drakken declares this trope, to which Kim replies "Fair enough".
On Jimmy Two-Shoes, after an episode ends on a particularly disgusting note, Jimmy and Beezy agree "This never happened."
My Little Pony: Friendship Is Magic, "The Return of Harmony, Part 2": Rarity says this word for word to her friends about the fact that she was brainwashed into thinking a large boulder was a giant diamond. Of course, Applejack brings it up very shortly afterwards.
Rarity: How in Equestria could [Rainbow Dash] think that tiny patch of cloud is Cloudsdale? Applejack: The same way he [Discord] got you to think that cheap rock was a bonafide diamond. Rarity: ...I thought we agreed never to speak of that again.
King of the Hill: Years ago, Hank, Dale and Bill "borrowed" Boomhauer's car for a joyride and droven it into a flooded quarry. The three then agreed never to mention it again. Hank had no choice but to break the agreement when the quarry was about to be drained, since they left clues linking them to the case.