"If I'm eating dinner with the hero, put poison in his goblet, then have to leave the table for any reason, I will order new drinks for both of us instead of trying to decide whether or not to switch with him."
Two enemies are sharing a drink and one of the glasses contains poison. At least one of them will attempt to poison the other by switching glasses while the other's back is turned. Common joke is either that someone poisons a chalice, then they get switched over, or someone knows he's got a poisoned drink, and tries to find a chance to dispose of it without being obvious, or pretending to swap the glasses, waiting for the enemy to really swap them and drink. A common trick (most famously seen in The Princess Bride) is to poison both drinks.
A less lethal variation is to shake up a bottle or can of soda or beer.
A Discredited Trope, more often parodied (or made fun of) than played straight these days. When switched and switched back, this is Two Rights Make A Wrong.
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Tantei Gakuen Q: Amped up to eleven; in order to trap the murderer who's planning to kill the last victim, they deliberately let him poison one of the coffee cups in a meeting with others. Then the detectives make an excuse so that everyone gets sent to another room. The coffee cups are then rearranged and re-served. Which one gets the poisoned one? Of course, the perp gets it, thanks to Megumi's photographic memory which remembers which cup is poisoned.
The long candle is set to burn extra hot and extra fast, but this backfires because since the contest is to put out the flame, Gon is able to set the candle down unguarded and get his opponent's candle much more easily.
Nakamura, a science teacher in Nichijou who's interested in Robot Girl Nano tries to see if she's susceptible to tranquilizers by giving her drugged coffee... only to realize too late that she'd poured and drank her own coffee after putting the tranquilizers in the pot. Later, she prepares this trick again, only for her adding the tranquilizers to make the coffee cup overflow. She instinctively leans forward to drink the excess off the top and... well...
A TV commercial announced that, on Mondays, a DVD with episodes of a British crime show would be included when you bought the Swedish evening paper Expressen. In the commercial, a British woman is preparing tea for herself and her husband, and secretly pours poison into his cup.
Husband: [Drinks his tea] Wife (smiling): "I poisoned your tea..." Husband (also smiling): "Darling, I know! I switched the cups." Wife: "So did I." Husband: [Gasps, and then promptly dies] * The butler enters the room.* Wife: "Bury him with the others, James."
A Distant Soil: Colleen Doran uses this early on in her comic. Jason is offered some wine by the seemingly-kind Sere. He asks her to drink hers first, which she does. He then takes a few sips of his. One of Sere's servants then enters with a small metal tube, which Sere injects into her arm — it's the antidote. (Or inhibitor, or something—it's just drugged, not poisoned.) Cue Jason falling over.
In a flashback in Batman R.I.P, we see Bruce Wayne drinking with a little Asian man in the Far East. The man then reveals he poisoned Bruce's cup. However, the man himself begins to fall victim to the poison. Turns out, according to Bruce, "You blinked. I switched the cups. Force of habit."
The very first Spy vs. Spy cartoon in MAD featured the black spy and white spy both subtly disposing of the tea they were supposed to be having by tipping it onto the floor. Two cats see the tea and lap it up, and are shown dead in the final panel.
Nikolai Dante: Played With. Nikolai and his half-sister Anastasia, who typically kills people by poisoning them are having a drink together. Nikolai's crest warns him that his glass may be poisoned, so he makes an obvious effort to switch the glasses, at which point Anastasia remarks that, as an experienced poisoner, she would expect a wary individual to try and switch glasses, so she would logically poison her own glass, instead. Then, when Dante switches glasses again, she remarks that since she's immune to her own poison, anyway, she would logically poison both glasses.
Film - Animated
Played with in the Disney film The Emperor's New Groove, Yzma tries to kill Kuzco by having his drink poisoned. During the meal, Kronk, the one to initially poison it, gets the drinks confused and ends up putting the poison in all three of them. They pretend to drink theirs, but it turns out none of the drinks were poisoned in the first place — what he had put in them was a potion that changed Kuzco into a llama.
Inverted in the fantasy film Shrek 2, when Fiona's father the King brings two cups of tea into Fiona's room, one of which he had spiked with a love potion as part of a ploy to get her to marry Prince Charming. After talking for a bit, Fiona reaches for one cup, but the King nervously tells her to drink the other, making up a transparent excuse about one of them being decaf. Fiona obliges, having no reason to be suspicious of her father. At the movie's climax, it's revealed to the audience that the King had actually changed his mind at the last second, instead giving Fiona the untainted cup.
Film - Live Action
In Sherlock, Jr., the detective played by Buster Keaton accepts a poisoned drink, then politely offers it to the would-be murderer's accomplice (who doesn't know about the poison).
The Assassination Bureau: Ivan Dragomiloff sees the countess pour poison into his chalice. He spins both chalices on a rotating table and the countess refuses to drink since she does not know where the poison is. Dragomiloff drinks the non-poisoned wine and fakes death. 
The Princess Bride features a subversion. After going through a long I Know You Know I Know, Vizzini distracts the Man In Black and switches the goblets, then carefully waits for his opponent to drink first (since presumably the Man In Black would try to weasel out of drinking if he thought he was about to drink from the poisoned cup). Turns out the Man In Black poisonedbothchalices, revealing to Buttercup afterward that he had built up a tolerance to the poison used. Ironically, both sides of his I Know You Know I Know were correct — "I cannot choose the wine in front of you" and "I cannot choose the wine in front of me" — even though he was only using them to fish for a reaction.
Confessions Of A Dangerous Mind had a fairly straight use of the trope. Patricia brings Chuck Barris a tray with two drinks; Chuck's is poisoned. She loses her attention for a moment, then comes back and notices the tray has been reversed. While Chuck isn't looking, she reverses it again. And she winds up choking to death on poison. Chuck hadn't reversed the tray, he just moved the objects on the tray around to make it look like he had.
The Court Jester has an early parody of this trope, although it was trying to remember which cup was poisoned, rather than switching them.
It's only in the flagon with the dragon because they broke the chalice from the palace.
Stardust: This scene when the three remaining brothers, Primus, Tertius, and Septimus are having a toast to their quest for the magic ruby:
Bishop: To the new King of Stormhold. Whichever of you fine young men it might be.
[They acknowledge him and drink. The bishop's face contorts. Seconds later, he collapses, dead. The brothers look uncomprehendingly at their cups, and then eye each other suspiciously. Tertius breaks the atmosphere with a laugh, which turns into wheezing, then starts having convulsions, and dies]
Septimus:[to Primus] You- [Clutches his throat and chokes on the poison. Seconds later, he falls on his back. Primus' eyes light up in realization at the fact that he is the only brother remaining. Thinking this means he is now the king, he picks up the crown reverently and is about to put it on when....]
Septimus: Ha-ha-ha-ha-ha-ha-ha! [stands up, laughing] You really thought you were king!
Primus: You killed the bishop.
Septimus: No, Primus, I think you'll find that you killed the bishop by drinking out of the wrong cup. [Primus looks crestfallen] Oh look, once you've finished wrestling with your conscience I suggest you return to your chamber. Leave the quest for the stone to me.
In the book, all the brothers are Genre Savvy enough to only drink wine they themselves have poured from a sealed bottle. In the film, Primus refuses wine from Lamia at the inn, probably for that reason.
At the beginning of the Daredevil movie, the blind Matt is handed mustard instead of honey for his tea. As soon as the prankster turns his back, Matt switches the cups.
In the 1933 movie Roman Scandals, the Roman emperor's food taster (Eddie Cantor) is told "The one without the parsley is the one without the poison." But some busybody in the kitchen notices the missing parsley and puts it on.
Abbott and Costello sometimes did a variation: Lou gets a necklace from a mysterious woman, detective comes by and says a mysterious woman just stole a necklace, then Lou tries to hide it in Bud's hamburger so they don't get blamed for it. Eventually, Lou ends up eating it, just as the detective comes back and mentions there's a reward for it.
Turned Up to Eleven in The Naughty Nineties, where Costello's character ends up having a drink with the Big Bad. At one point Costello loses track of who has the poison, so he dumps it into a potted plant. That Poor Plant whithers away and dies, prompting Costello to wipe his glass out very thouroughly.
Happens to Lou again in Pardon My Sarong, when he and the island chief are having a "friendly" drink. Lou comes out on top of for this one.
Subverted in a terrifically cool way in the Hong Kong movie Color of the Truth. Evil Mob Boss visits Hired Mooks in their hideout bearing celebratory bottles of wine. Head Hired Mook suspects this trope, so he demands that Evil Mob Boss takes a swig of the wine first - which he does. Evil Mob Boss leaves the Mooks to their party, then rushes out to a trailer truck parked a short distance away - in which a mobile medical unit is waiting to pump the poison out of his stomach.
Used in multiple The Three Stooges shorts. In one notable instance, Shemp and the villainess take turns distracting each other and swapping their wine glasses. The Genre Savvy villainess, however, simply taps the glasses together when it's her turn, causing Shemp to swap the glasses around again and give himself the poison.
In the Czech film Císařův pekař - Pekařův císař a group of conspirators attempt to poison Emperor Rudolf II. The plot is derailed when the chalices of wine - including the poisoned one - are used to demonstrate Copernicus' heliocentric system.
In Scott Pilgrim vs. the World, Scott pulls this: after the vegan bass player Todd bests him in a bass battle, Scott attempts to reconcile with him by offering a coffee with soy milk in it. However, as Todd has Psychic Powers as a result of his veganism (yeah, we know), he knows Scott's actually "poisoned" the cup with half-and-half, so he takes the one Scott is going to drink... but it turns out Scott offered him the one with soy milk and held the non-vegan cup close, and merely thought really hard about doing it the other way. The vegan police then come and take away Todd's powers for breaking the code (yeah, we know).
In Pirates of the Caribbean: On Stranger Tides, Jack Sparrow lies about which chalice contains the mermaid tear, and thus will grant life, while the other one will drain life. Blackbeard values his own life more than that of his daughter, drinks the one he thinks will give life, and ends up dying.
At the end of The Little Girl Who Lives Down The Lane, as Rynn is making tea for herself and Frank, she puts potassium cyanide in her own cup. It's not clear if she anticipated that Frank would be suspicious and switch, or if she was initially intending to kill herself rather than submit to Frank's blackmail (and possibly framing him with her murder). But either way, Frank becomes suspicious and demands that they switch cups, so he ends up dying.
Played with in the 1997 Prince Valiant film. The villain spikes Princess Ilene's goblet with some kind of potion (love potion, sleeping potion... wasn't made clear). Ilene distracts him, switches goblets, and pours candle wax into his goblet. The villain get suspicious and switches them back. He drinks first, and as he chokes on the wax, Ilene escapes.
In the 1938 German movie Napoleon ist an allem schuld ("It's All Napoleon's Fault"), Lord Cavershoot (Curt Goetz) and Lord Cunningham (Paul Henckels) spike one of the cups for afternoon tea with a delayed Mickey Finn so that the intended victim will be asleep and thus miss the big party in the evening. Unfortunately for them, while they aren't looking, Lady Cavershoot discovers an unsightly spot on the tablecloth, which Cunningham caused when he poured the potion into the tea. She doesn't know about the potion, but because she doesn't want the guest to face the spot, she quickly turns the table around, accidentally causing Lord Cunningham to drink the Mickey-laced tea.
In Heathers, Veronica mistakenly picks the wrong of two cups to give to Heather, who then dies from drinking the lethal potion.
Once upon a time, an old man got tired of the neighbor kids mischievously stealing apples from his big apple tree. So he takes a sign, and writes on it: "One of those apples is poisoned!!!". He considers the matter done and goes to sleep. The next day, he finds a note attached to his big sign, which says, "Ha! Now there's two!"
In the Sherlock Holmes adventure A Study In Scarlet, the killer uses a variant to get revenge on the people who kidnapped his fiancee, without directly killing them. He corners each victim and produces two pills, forcing the victim to choose one and he agrees to take the other. The twist is that he deliberately has no idea which is poison, but trusts in fate to punish the guilty. This breaks down when one victim refuses to pick, so the killer just stabs him.
In the Cadfael novel The Potter's Field, a dying wife challenges her husband's mistress to a poisoned-chalice wager in which neither knows which glass of wine is drugged. If the wife dies, the mistress will be free to marry her husband; if the mistress dies, the wife swears to punish herself by refraining from taking herbal painkillers for her illness. The mistress agrees; by chance she's the one who dies, and the wife makes good on her promise.
Used in Brian Jacques's Redwall novel Marlfox by Lantur, on her mother Silth. The mad old queen either overthinks it by several layers, or by one layer too few, depending on your point of view, on trying to figure out if her own royal goblet was poisoned or the plain pewter one intended for Lantur. She forces Lantur to drink from the pewter cup in the end, but the poisoned one was indeed her own royal cup the entire time.
It also occurs in Outcast of Redwall. Swartt convinces Bowflegg that the wine isn't poisoned by drinking some of it straight from the bottle. That's because the poison is actually smeared onto the rim of the goblet that Bowflegg drinks from.
The trick is pulled at least once more, and is later subverted by a Dangerously Genre Savvy fox who realized the trick (he's gotten rid of in a less direct way).
In the novel The Sherwood Ring, Barbara is taken prisoner by Peaceable, and lets him know (she can't hide it, really) that she's got sleeping drops on her person after hitting the apothecary that morning. After he starts to get a crush on her, and lets her know about it, she decides to pull the chalice switcheroo and hands him the red glass. He's well aware of what she's doing (especially since the cups are of different colors) and points out to her that he doesn't blame her for trying, it's just the general stupidity of her trying that irritates him. He switches goblets and challenges her to drink hers, she does, he does...then he says, "It was in the green glass all the time, wasn't it?" Yup. He'd informed Barbara earlier that all the girls he ever met in his life came off as fools and he assumed when she pulled out the goblets that she wasn't as smart as she appeared, and she took advantage of it. He promptly proposed marriage before he passed out.
Nation, by Terry Pratchett: Half-averted, half-subverted. In order to prevent two men from killing the islanders (and after they had already killed one), Daphne gets them alone and poisons them with undiluted beer. Foxlip, the one who had killed, does not switch the cups as per the trope, instead having Daphne mix the drinks together so they all get the same. Like Princess Bride, all the cups were poisoned. Unlike Princess Bride, it wasn't that Daphne was immune so much as understood the trick to make the beer safe: Spitting into the beer to neutralize the poison, and singing the ritual beer song to count time it takes to work. Daphne did this right in front of the men and asked they do it too, knowing that they would never partake in "pagan mumbo jumbo." As a result, Foxlip drank and died. The other man survived only because of continued suspicion but was sent running soon after.
In Robin Hobb's Farseer trilogy, FitzChivalry ostensibly poisons his "victim"'s glass, and gives him his own. He later takes a few sips of that unpoisoned glass, before noticing, a little too late, that the bottle had been poisoned by the Evil Prince. He barely survives, the victim doesn't.
In the Agatha Christie short story Accident, a man suspects a woman of being a murderer. Before he can be certain, he must take a drink she offers him. He waits for her to drink first but doesn't notice that she pours her drink into a plant.
In Curtain, 20th century Iago Stephen Norton has manipulated Barbara Franklin into trying to poison her husband. While everyone else is looking at meteors, Hastings, completely unsuspecting, swivels the table around to get at a book, and Barbara drinks the poisoned coffee.
The G. K. Chesterton short story "The Bottomless Well" uses exactly the same variant as Curtain, with a revolving bookcase to accidentally switch the cups.
In Death: Non-lethal drug example. Whenever Eve goes too long without sleep in the middle of a case, Roarke will usually try to push food and sedatives on her. On one occasion, she switches their bowls of soup with a snarky comment and starts to eat — only to fall asleep. Roarke makes fun of her for for it before she goes under.
In The Count of Monte Cristo, there is some in-story discussion of this trope as used by the Borgias. According to one of the men, the chalice contained a secret compartment that released the poison when the cupfiller needed, thus allowing him to serve an entire row of cardinals with only one in the middle one dying.
Mort: When Mort, who is subbing for Death, visits the Agatean Court, a poisoned piece of squishi goes back and forth between the vizier and the emperor's bowls as each thinks of a better reason for the other to eat it.
In the Benjamin January series, it's used to Shoot the Dog. There's no way to arrest a murderer without his revealing an extremely damaging secret, so Hannibal Sefton, who's a potential threat to the man, deliberately goes to have a drink with him and switches glasses when the man's back is turned. The murderer dies of his own poison and the secret is safe.
Richard executes this successfully in Oblivion with two identical cars, one of which contains a bomb.
Live Action TV
In the Doctor Who episode "Boom Town" (9th Doctor), the evil Margaret Blaine distracts the Doctor during dinner and slips poison into his drink. Then the Doctor, still smiling and without missing a beat, deliberately and in full view switches the glasses.
Frasier had a variant in one of their end-of-episode silent sketches. Roz brought some coffee to a woman she disliked but then got a call on her mobile; while she was distracted, the other woman switched the coffee. Then she got distracted, so Roz switched them back. This cycle of distraction and switching continued for some time until at the end, Roz just poured both cups down the sink.
In Police Squad!! a woman poisoned one of two drinks. It worked, but I'm sure she drank the one with the poison with it. This may have been on purpose; after all, Police Squad!! was famous for never letting a go by without a joke, and packed tons of subtle jokes in-between the over-the-top sight gags.
In Family Matters, Extraverted Nerd Steve Urkel sits next to a couple of bullies in the cafeteria, then has to leave the table for a moment. They sneak heavy amounts of salt into his lunch. Suspecting that they probably did something to his food, he uses a Look Over There ploy to make the bullies look away. They sarcastically play along, and Urkel picks the trays up and puts them down without switching them. They then use a Look Over There ploy themselves ("Look, it's Steven Hawking!"), which Urkel plays along with more convincingly, allowing them to switch the trays "back". Urkel then takes the "unpoisoned" tray and leaves before the boys bite into their salty lunch.
Skithouse: The inviting host enters with one, and only one, glass of wine on a tray. He puts it down, lays a distraction, adds the poison, and he and his captive play out the distract-then turn scene, while the single glass is in the center of the tray and not moving at all. In the end, the host picks it up, and drinks.
Saturday Night Live: Steve Martin, always eager to protect his status as hosting the show more times than anybody else, once did a skit where he attempted to maintain his status by poisoning Tom Hanks before he could tie the record, by slipping poison from his ring into Tom's whiskey. A series of switcharoos took place, before Tom Hanks pulled a fake switcharoo. Steve Martin, now suspecting that, decided to throw out the whiskey and order champagne, which he again poisoned. After another couple switcharoos Tom resolved the issue by punching Steve in the jaw. Only then did Steve find out that this was only Tom Hank's fifteenth time hosting, meaning he was still one short of tying Steve. Both of them felt very foolish.
House: Played almost-straight when House buys two coffees, grinding up amphetamines in one to give to Wilson. Being Genre Savvy, House offers the clean one to Wilson, who suspiciously declines House's offer and takes the drugged one from his desk.
Seen in a Morecambe And WiseHamlet parody in which Eric and Ernie resort to ever more desperate measures to distract each other as they switch the poison chalice concluding with; Ernie: Look, there's a naked lady. Eric: Look there's an even nakeder lady.
Dracula: The Series: A variation; the involved parties are both vampires and the "poison" is holy water. The exchange happens right in front of the poisoner who doesn't notice due to Dracula's fast reflexes.
Played straight in Merlin (except that they're both trying to get the 'poison', to save the other) up until the point where Arthur distracts Merlin to put both the 'poisoned' drink and the safe one in his own goblet. He drank it, but there was actually no poison in it, just a sleeping potion.
An episode of CSI: Crime Scene Investigation does this, with peanut butter. (The intended victim was highly allergic.) The murderer changes her mind and takes the 'poisoned' cup but the victim ends up with it anyway. Just as well that what actually ended up killing him was a bee sting.
On Scrubs, Turk informs Dr. Kelso he's giving Carla regular coffee instead of decaf, due to her proclamation that the two of them will be going to bed at the same time, early. Dr. Kelso listens sympathetically, being an old hand at drugging one's spouse himself. Then, when Turk looks away, he switches Turk's decaf for Carla's caffeinated. Cut to: Turk awake all night as Carla slumbers.
Dr. Kelso: "What a fun day..."
On I Love Lucy, Ricky tries to slip Lucy a sedative to calm her down, but Lucy thinks he's trying to poison and kill her, which leads to...
Lucy: I switched glasses!
Ricky: I know you did. I switched them back.
Get Smart. Max is dining out with a female KAOS agent. She is trying to give him a knockout pill, while Max is attempting to spike her drink with truth serum. Both of them keep pointing out things to look at behind the other's back, so they can switch drinks. It ends with Max telling everything he knows to the KAOS agent, who is fast asleep.
In The Two Ronnies serial "Done To Death," the two detectives suspect Blanche has poisoned their food. They both make several attempts to change their plates with hers; in the end, they both think they've succeeded and start eating, only for Blanche to remark innocently that she's got her original plate back. Fortunately, the food wasn't poisoned in the first place.
Done openly in Sherlock: the serial killer has two identical bottles of pills, one poisonous and one completely harmless, and forces his victims at gunpoint to choose one while he takes the other. Sherlock realizes the gun is fake and is about to simply walk away, but the killer challenges him into playing anyway. However, before they can actually take the pills, Watson shoots him, and we never do find out which was which.
Assuming the killer was telling the truth a variation is created. No matter how clever Sherlock is he cannot figure out where the poison is because the killer doesn't know and has just been lucky. Naturally Sherlock doesn't believe him and gets sucked in.
Chespirito has done it as the Chapulin Colorado and in historical or mythical sketches
On 3rd Rock from the Sun, a reporter had to be Killed to Uphold the Masquerade. Dick poured two cups of brandy ("No thanks, I'm driving.", "Oh, this is special driving brandy.") and poisoned one of them. Then Dick dramatically switched the cups around a number of times. After a beat:
Dick: You look thirsty, why don't you drink both?
A variation occurred in Boy Meets World. Sean and Corey decide to mess with Dingus's meal by placing a ton of salt and a huge amount of cream sauce inside Dingus's burger while he left the table. Dingus was apparently Genre Savvy enough to suspect that the did it when he returned to the table, and then told them to look elsewhere while he lifted the plates up and down. Corey and Sean likewise suspect he switched the plates, and do the same method to get it back to him, only to realize that Dingus had in fact faked them out, and that they ended up getting the messed with meal that was intended to be for Dingus.
Played quite straight in the German comedy "Klimbim": 1st round, the heroine switches the chalices, 2nd round, the villainess switches the chalices, 3rd round, the heroine FEIGNS to switch the chalices, 4th round, the villainess switches the chalices. Glug Glug Glug. Hey, you should drop dead now. Drat. Since the villainess isn't completely stupid, she does NOT drink her glass, but don't worry, she gets what she deserves eventually.
The Prisoner - Number Six is made to believe his aggressive behavior has been neutralized by ultrasound brain surgery - he comes to realize it had been staged and he was being kept passive with drugs, at which point he switches his drugged tea with a cup the scientist in charge is taking with him.
FoxTrot had a variation with salt in milk, between Jason and Peter with multiple switches and some alleged fake switches, culminating in the line "We're trying to figure out which of us should be throwing up right now." Turned out Paige had somehow gotten the salt.
A Garfield strip has Jon Arbuckle suggesting that he and his date drink from each other's glasses... on the grounds that his last date tried to poison him.
Clue VCR Mystery Game: This happens multiple times during the dinner scene. It reaches the point were no one ends up eating anything at dinner as no one is sure what has been poisoned and what hasn't.
Older Than Steam: William Shakespearedid it in Hamlet. Specifically, it was done with swords. Laertes cut Hamlet with a poisoned sword in a fencing match, and in the ensuing scuffle Hamlet took Laertes' sword and cut him as well. In the exact same scene, Gertrude (accidentally or not, depending on the director) drinks the poisoned wine Claudius meant to use on Hamlet if the sword trick failed.
Bullshot Crummond. Bullshot suggests one last drink before Count Otto von Bruno kills him off, and slips knockout pills into his glass. First he hides the doped glass and holds out the other to Otto, knowing he'll insist on swopping. Otto takes a sip and spits it out, then suggests they drink "Eastern European fashion" so Bullshot gets the doped glass. Both end up keeling over.
In Discworld, you poisoned someone with a truth serum, and both characters distracted each other with pictures in the inn while switching the table. You had to create a new picture to get your opponent to drink it.
In Suikoden II, the King gets poisoned not because there was poison in the cup or the wine given to him by Jowy; but because Jowy had poisoned himself before he symbolically bled himself into the wine to symbolize his loyalty. He then took a lot of antidote, but still almost died.
While not being entirely 100% on the Trope, there is a story in a book in the Elder Scrolls games which recounts the story of a noble man inviting his most trusted followers to a great meal. Most attendees feared that the food was poisoned and thus faked eating it. Once the meal was finished, the lord said that instead of the meal, the cutlery was poisoned, but if attendees who deserved to be poisoned would confess of their wrongdoings against him, they would receive the antidote. One man admitted consorting behind the lord's back and received the antidote. At this point the lord admitted that the meal and drink were perfectly fine and the "antidote" was actually an incurable deadly poison. At the end of the book the writer deduces that the ploy was more to inspire fear into the lord's servants than expose any spies and requests from his employer to be resigned from his position.
You actually meet the nobleman in question in the expansion pack, Tribunal - King Helseth.
This comes up in party banter in the Dragon Age IIMark of the Assassin DLC. Varric mentions a relative who tried to do this but was very bad at intrigue. "Wasn't finished gloating to his enemy when he keeled over. I think the family always had a sense for terrible drama."
In Kingdom of Loathing, at the end of the 'Strange Leaflet Quest' you meet a giant who offers you two shotglasses of potion; he tells you that one is poisoned and if you drink the safe one, he'll take you to fairyland. It's a Shout-Out to Ender's Game — as in the original, the giant is lying, both potions are poison, and you have to Take a Third Option to solve the puzzle.
In Dishonored, you come across one of your assassination targets just as he's about to poison a political enemy. You can then switch their glasses and have the target drink his own poison.
In Oglaf, the obviously gay Xoan ambassador who is friends with the Apprentice's Mistress offers the Apprentice poisoned food claiming it's all the rage and pours the antidote over his penis in hopes that the apprentice will have to suck it off. The Apprentice says he's going to his room to write a letter to the Mistress saying the Xoan ambassador murdered him. However, when the Ambassador tries to turn it into a mystery drama, including offering a locket with the ambassador's picture inside (which he has a bag of), the apprentice says he doesn't believe it was poison. Later the Mistress calls the Apprentice in to punish him for being in the room that the ambassador had called him to; only for him to collapse from poison causing the ambassador to yell "He's been poisoned!". Later still, when he's sent to be cured in an painfully disturbing way, the ambassador reveals the Apprentice made a smart move because the antidote was poison as well.
Done in a Looney Tunes short where Bugs and Yosemite Sam keep swapping drinks. Bugs eventually starts the whole table spinning. Sam stops the table and forces Bugs to drink at gunpoint. He still gets the wrong glass.
In an episode of Star Wars: The Clone Wars, Obi-Wan and Anakin use The Force to pull this trick off on a gang of pirates. But then they get knocked out anyway between episodes. Because the lead pirate's pet sees them and uses a gas to knock out everyone.
A rumor to the origins of toasting is thought to contain this trope. It was more a sign of trust (similar to handing a weapon business end pointed at you to someone) that your drinks could accidentally slosh and mix, and both men would come out of it in good health. Whether it is Truth in Television or not is heavily debated.