In Full Metal Panic!, Sōsuke is a lousy liar. Every time he tries to lie to the other students at school about how he's A PERFECTLY NORMAL BOY WHO LOVES DOING NORMAL, BORING SCHOOL STUDENT THINGSAND HAS NOTHING TO DO WITH THE ARMY, they're hardly ever shown to be convinced. And he fails at lying when it goes the other way and tries to convince them that he's actually a big, bad inhuman terrorist that's unreasonable and will kill anyone. Interestingly enough, however, when he's lying to a corrupt policeman who's actually trained in interrogating people, he actually manages to convince him about his phony background story (wanting to fight in the Yami Battle for money and power, and also to buy back a prostitute in Tokyo).
Also Luffy, who has absolutely zero ability for deception...pre-Time Skip, anyway. Afterwards, he has a modicum of ability, just enough to barely deceive those who are not completely oblivious or naïve...under certain circumstances...after his crew helps him come up with the lie...
And let us not forget why Sanji wanted to eat the devil fruit that grants invisibility.
Sanji: If I ever got the ability I wanted to use it to GIRL'S BATH. Ah no, I meant I asked myself GIRL'S BATH. No, I wanted to use it to benefit the humanity GIRL'S BATH.
Nagi of Hayate the Combat Butler has been known to fall into this. Sending Hayate out with Maria on a false date to throw off a stalker. The excuse she gives for not taking Hayate out herself (despite having fallen in love with him at the start of the story) is that they would be mistaken for just a sibling pair. Even normally clueless Hayate comments on how transparent the excuse is.
Also, Haruhi herself generally manages to be a pretty terrible liar, especially when it involves things relating to her feelings for Kyon. Luckily (or perhaps unluckily, depending on the way you look at it) for her, they either pass right over his head or he is very, very good at turning a blind eye.
In the anime version of Bokurano, Yosuke Kirie's pilot mark appears on his face, unlike with most of the other pilots, who could hide it under their clothes. He tells his mother that he hit his head, and Ushiro says she would never believe such an obvious lie.
Ruby from Rave Master, because he has a bad habit of blurting out whatever he is thinking.
Editoral Section, NOT Comic Strips
Calvin from Calvin and Hobbes. His lies tend to outrageous even when you consider that he's a little kid. Trying to blame Hobbes every time he does something bad is a common one, but there are lots of others. Really, the only thing sillier than his lies is that he actually expects anyone to believe them. (As his mother said once, "just how dumb do you think I am?"
Not just Calvin. Susie is a Bad Liar too. When Calvin decides to cheat on a math test by asking Susie for answers, and when she tells him 12 + 7 is "a billion," even the math-impaired Calvin senses something is up: "That's what she said 3 + 4 was."
His dad is pretty bad too, although he can usually fool Calvin. One example:
Calvin: Dad, were their dinosaurs when you were a kid?
Dad: Sure! Your grandfather and I used to put on leopard skins and brontosaurus for all the clan rituals.
Mom(after Calvin has left): Listen, buster, I think Calvin's grades are bad enough, don't you?
The nighttime monsters that live under his bed are lousy liars too. Every time he shouts out something like "Any monsters under there?" he's sure to get several replies of "Nope!" or the like. Even Calvin is smart enough not to fall for that.
Most Certainly Not Fan Works
Elspeth of Luminosity is an interesting example: a part of her power is that when she tells the truth, anyone she's talking to knows absolutely that that is the truth. Because she lacks the not-quite-Required Secondary Powers, this makes it glaringly obvious when she's lying because the heavens don't part to sing that this is true.
In Oh God, Not Again!, Ginny is specifically portrayed as being this. When Ron wonders where Ginny got the extra money she was spending on school supplies (which she got from her cut of a completely inaccurate book by Lockhart), she freaks out and demands to know if he's implying she stole it. Harry thinks to himself that the Weasleys were raised to be honest.
In The Man With No Name, the (10th) Doctor laments his inability to lie convincingly in this regeneration several times.
Gilda lies constantly, and never stops to consider if she's contradicting previously established facts. Eventually, even the prosecutor gets fed up with it.
No Chance This Could Be Animated Films
In the teaser trailer for Arthur Christmas, an elf found in the North Pole by a wildlife documentary filmmaker goes into detail about every single thing that happens in the trailer while saying that they're not, such as how he's "not" an elf, the man in the sweater who pops up and calls Santa his dad is "not" the son of Santa Claus, and that the North Pole is "not" a place where Santa makes test flights so that he can deliver toys to millions of children around the world while a giant red flying saucer that serves as Santa's sleigh flies right over them, Also saying that there is no movie.
In Madagascar 3, when the main group gets on a circus train, they lie and say they are circus animals. Alex tries to lie about performing a technique, and when Melvin, Gloria, and Marty randomly begin individually suggesting things, Alex has to mix everything together into an impossible mess where he says he high jumps into a pool of Cobra's and pulls up with a jetpack and releases balloons to all the children of the world. Hilariously, the Circus animals believe him despite the insanity of the trick Alex is describing. Ironically, he does that very trick in the movie's final scene.
Definitely Not Live-Action Films
Jimmy Durante in a scene in Jumbo attempts to sneak his beloved elephant Jumbo off the circus grounds, only to be confronted by a sheriff, who demands: "Where you going with that elephant?" Caught red-handed, Durante blithely replies "What elephant?"
There's another Jimmy Durante movie in which he's guarding the dressing room of the beautiful young singer played by Kathryn Grayson from a pushy fan. "She's got twelve kids and I'm the youngest!"
Miracle Max: You got any money? Inigo Montoya: Sixty-five. Miracle Max: I've never worked for so little. Except once, and that was a very noble cause. Inigo Montoya: This is noble, sir. His wife is... crippled. His children are on the brink of starvation. Miracle Max: Are you a rotten liar. Inigo Montoya: I need him to help avenge my father, murdered these twenty years. Miracle Max: Your first story was better.
The last two lines are either an inversion, in that Miracle Max thinks he's lying when he's not, or it's simply that Miracle Max is less pleased by the real story than he was by the lie.
The titular sorcerer actually says this is a good thing, as good wizards are supposed to be truthful. This doesn't stop him from lying, although he's not that great at it either (sake is not Chinese).
Actually he succeeded in his lie. He came off as a dumb flatfoot trying to explain away a dragon in the middle of the city, and the policemen arriving on the scene believed him. The magical disguise probably didn't hurt.
In A Guide For The Married Man, Robert Morse is instructing Walter Matthau how to successfully cheat on his wife. A series of vignettes are used to illustrate different points. One directive concerns what you do if you are caught: Deny, deny deny!. The vignette is a man (Joey Bishop) who is caught in his own bedroom by his wife (Ann Morgan Guilbert). She keeps asking him what he's doing?, what's going on?, who's that woman? He keeps answering with denials: Nothing. I don't know what you mean. What woman? All the while, the woman is getting dressed, and eventually leaves, he makes the bed and straightens out the bedroom, and finally goes to his favorite chair, lights his pipe and picks up the newspaper. When he's all done, there's no indication that anything out of the ordinary ever happened. She eventually is so disconcerted that she simply asks him what he wants for dinner.
In Star Trek: First Contact, Data is... simply... imitating the behavior of humans when he gets his new arm of flesh slashed during an escape attempt from the Borg Queen's clutches. Queenie then tells him that he has now learned how to lie. Data's fortunately a very quick learner and outgrows being a Bad Liar to pull off the ultimate fakeout on the Queen later in the movie.
A classic example occurs at the end of Animal House; as the homecoming parade descends into chaos, Kevin Bacon's character, with a forced grin, shouts "Remain calm! All is well!" This later turns into desperate screaming, before, eventually, he is literally flattened by the advancing crowd.
This Really isn't the Literature section.
Animorphs: When called upon to make up a phone number, Cassie comes up with "1234-5678". Marco, who has no problem making up anything ever, chastises her ASAP.
Ax at one point is convinced to take the blame for giving humans the morphing technology to spare his brother the shame, but fails to think up a decent lie about it. When his superiors later ask how he met the Animorphs he replies with the truth: They rescued him from his ship at the bottom of the ocean...using technology he supposedly gave them. No one buys it.
In the Aubrey-Maturin series, Stephen Maturin ponders Sophie Williams' talent for prevarication.
Post Captain: She lies with as much skill as Preserved Killick — a desperate stare, and her face the most perfect damask rose.
Sophie Williams is Jack Aubrey's fiancée at this point.
Preserved Killick is the name of Jack Aubrey's servant. Really.
Lori is most successful at avoiding issues by not mentioning them at all than by telling a lie. In Aunt Dimity: Detective, she jokingly asks Nicholas Fox to teach her to play poker, since he so much better at concealing his feelings than she is.
In Aunt Dimity and the Lost Prince, curator Mile Craven acts suspiciously when Lori and Bree ask after his former employee Amanda Pickering; his gaze drops and his chatty demeanor abruptly disappears. His discomfiture is related to his inability to keep a secret.
Billy Bunter of Greyfriars. "I say, keep that beast Coker off! I wasn't in his study when he found me there, the suspicious beast! I wasn't after his cake! There wasn't any cake, and I never touched it, and I had hardly a mouthful when the brute came in! I say, you fellows — ow! Oh crikey!"
"What makes the rear-rank breathe so 'ard?" said Files-on-Parade. "It's bitter cold, it's bitter cold", the Colour-Sergeant said. "What makes that front-rank man fall down?" said Files-on-Parade. "A touch o' sun, a touch o' sun", the Colour-Sergeant said.
In Jingo, Sergeant Colon and Captain Carrot are both really bad at going undercover. In fact, Carrot is such a bad liar in Jingo that he can't even convincingly keep quiet when someone else is lying. During a briefing at the palace, the Patrician sends him out of the room because watching him twitch every time Commander Vimes opens his mouth is disconcerting.
The Dresden Files. Harry Dresden admits to being a bad liar (especially in the earlier books), mostly by virtue of just not being able to make his lies sound natural. As the series progresses, he gets somewhat better at being deceptive, mostly by being very selective about what information he imparts.
In Cold Days, Maeve gains the ability to lie after being corrupted by Nemesis and hinges her plan on the fact that faeries, in all known circumstances, Cannot Tell a Lie. This would be brilliant, except that she has no experience with lying and isn't very good at it, causing her to accidentally say a few things that tip Harry off to what's really going on.
Harry Potter. As Snape said, he "has a tendency to wear his heart on his sleeve." He gets better, though. Naturally, the fact that Snape is an accomplished legilimen (telepath) does not help in this instance.
A more straight example is Hagrid. In Order of the Phoenix, Hagrid was briefly questioned by Dolores Umbridge regarding some footprints in the snow. Although he tries to defend Harry and friends by lying about them, it's pretty clear that he's doing an absolutely bad job at it. It still managed to get Umbridge away, though.
Wanderer from The Host, mostly because Souls (with the exception of Seekers and members of the first wave) never needed to lie.
Huckleberry Finn: Huck is occasionally brilliant at spinning yarns to get what he wants. More often, however, he forgets his fake identity, where he's supposedly from, and what he claimed his job was. Hilarity Ensues.
Michael from the Knight and Rogue Series doesn't like to lie, but when he gets into a situation where he's really got no other options, or if he's trying to spare someone's feelings, he'll give it a shot. Since he's so rotten at it though, it makes no difference. However, in a really desperate situation he can convincingly lie, and because he's known to be so bad at lying no one doubts him in the slightest.
Alek from Leviathan. Literally every time he opens his mouth to try and tell a lie, he effs up. It doesn't really help that the people he tries to lie to are really sharp.
Christine in The Phantom of the Opera, which makes it impossible for her to convince Raoul and her adopted mother that she is not in any danger from her stalker, and later makes it impossible for her to hide from Erik the fact that Raoul and the Persian are inside his house.
The protagonist of Rumo & His Miraculous Adventures is this. When he asks the mentor for help, he tries the "my friend Urs has this problem..." approach, but slips and says "I" instead of "Urs" several times. The mentor is nice enough not to mention.
In the Honor Harrington books, Honor, Sir Webster, and a few other Manticoran officers-cum-diplomats are this. In fact, that's part of the reason they're chosen as diplomats; their inability to lie well makes their honesty equally obvious.
In fact, Webster is such a terrible liar that when he tries, people think he's just being sarcastic.
Live-Action TV? That's... somewhere else
In Smallville, Jimmy Olsen is the worse offender, but Clark Kent is also pretty bad. Lana Lang and Chloe Sullivan to a lesser degree.
In the Red Dwarf episode where they visit the alternative reality, Holly is seen covered in lipstick after meeting Hilly. When asked "What is that lipstick on your face?", his reply is "What face?"
And in a different episode, Lister has to teach Kryton how to lie.
In the episode "Out of Time", Kryten learns what will happen to the crew in the future. He is upset that Lister is destined to become a brain in a jar, but can't tell him anything about it. He starts to cry in the kitchen when Lister comes in:
Lister: Everything okay? Kryten: Oh, yup yup. Those darn onions get you every time! Lister: What onions? Kryten: Ah, the onions I'm about to peel. I always get a little emotional when I have to deprive an onion of its skin. Lister: Don't Nixon me, man! Tell me the truth!
Phoebe: Ooh, honey. You stink at lying. Joey: I do not. Phoebe: Oh really. OK, let me ask you something. Yesterday at the coffee house, I went to the bathroom and when I came back, my muffin was gone. Who took it? Joey: Somebody opened the door to the coffee house and a raccoon came running in, went straight for your muffin, and I said, "Hey don't eat that! That's Phoebe's," and he said... He said... "Joey, you stink at lying." What am I going to do?
Also in Friends, when Rachel was keeping her pregnancy secret. In the Season 7 finale, the audience wasn't supposed to realize Rachel was pregnant until the very last minute, so she lied incredibly well, even in spur of the moment situations. But then in the Season 8 premiere, when the audience does know Rachel's pregnant, her lying skills go straight down the toilet and the secret is out in no time.
An odd Reality Television example is Zoe Zanidakis from the fourth season of Survivor. When Kathy asks her if she's going to vote Kathy out, Zoe pauses for over ten seconds. Then she says no. Oops.
Ned in Pushing Daisies. When asked how he worked out that the Victim of the Week's plane was hijacked just from looking at his body (when what he actually did was resurrect the victim and talk to him), he said "DNA...ish?" In another episode, he and his cohorts pretended to have been sent by the Vatican, and when a nun said that she thought they'd be Italian, his response was "We are. Part time."
Smart:(Makes an impressive sounding, but very absurd claim.)
Listener: I find that hard to believe.
Smart: Would you believe (insert a less impressive, but still abusrd claim)?
Smart: How about (insert a claim that sounds pathetic)?
In the Firefly episode "Jaynestown" Simon has to pose as a buyer of mud. Luckily for him, the foreman has not one grain of paranoia in him, as he's very, very unconvincing. This elicits the remark by a crewmember: "Who is this diabolical master of disguise?"
In the movie Serenity, he does a better job impersonating an Alliance inspector. Which is odd, because technically that scene came first for the character.
Plus, he had time to prepare before infiltrating the Academy. Mal designated him as their buyer cover about five minutes before he had to perform. Simon does not do that well on the spot, but give him prep time, and he's pretty gorram amazing.
Perhaps because he's never thought of buying mud, and has little to no idea what is done with it, while he's very good at the medical jargon used at the Academy.
In the episode "Ariel", Jayne tries to convince the hospital authorities that he's an ambulance driver. After a heap of coaching, he answers the question even though it's never asked.
Saturday Night Live had Tommy Flanagan (Jon Lovitz) and his wife, Morgan Fairchild... whom he's slept with.
A perennial flaw of the Unsympathetic Comedy Protagonist. Basil Fawlty of Fawlty Towers and Father Ted are two major examples. In their case it's to get out of trying situations, only the explanations become so convoluted, bizarre and obviously fake, they needn't have bothered.
Rene in 'Allo 'Allo! is also a terrible liar, never more than when trying to explain to his wife why he's embracing one of the waitresses. She however seems to accept his nonsense — whether she genuinely believes it or just wants a quiet life is never clear.
Buffy: You know, I just woke up, and I looked in the mirror, and I thought, hey, what's with all the sin? I need to change. I'm dirty. I'm bad with the sex and the envy and that loud music us kids listen to nowadays.... Oh, I just suck at undercover.
Buffy has this problem in other episodes. Example, from "Lie to Me":
Buffy: Um, uh... there was a-a cat. A cat here. And um, uh, then there was a-another cat. And they fought. The cats. And... then they left. Ford: Oh. I thought you were just slaying a vampire. Buffy: What? Whating a what?
And then there's Anya trying really hard to convince the Watchers' Council that there isn't anything demonic in her origin, no sir.
Early on, Willow got this role before basically passing it on to Anya.
Then there's Tara. When Spike gets the Buffybot from Warren to use as a sex toy, Tara ultimately decides to tell Dawn that he made it with the intent to play checkers with it. Dawn, of course, isn't fooled for a second.
Also, when Faith tries to frame Buffy to Giles for something she did (can't remember what), Giles only pretends to be mad at Buffy until Faith leaves, at which point he reveals he knew Faith did it, since lying isn't one of her strong points.
Over on Angel, according to Gunn, lying is the "one" thing Fred isn't good at.
Normally, Dean Winchester of Supernatural is an excellent liar. He impersonates FBI agents, police officers, park rangers, what have you. But when Dean contracts a Ghost Virus, and becomes a complete and total coward, he also turns into the worst liar north of Mexico.
The title character of Monk is shown to have this character trait.
In "Mr. Monk Meets the Playboy," when Sharona asks him if he actually saw the photos of her posing nude; he hesitates for a long time, tentatively says "No," then blurts out "Yes" as he's walking away.
In "Mr. Monk and the Red Herring," when Monk first meets Natalie, he blurts out that she's taking birth control pills in front of her daughter Julie, and upon realizing the implications tries to claim he was mistaken and they were really "adult tic-tacs". Natalie later tries to have Monk pose as an expert on fish during an argument with one of Julie's teachers, which doesn't go any better.
In Misfits, Nathan and Simon explain their friend Curtis's absence to his girlfriend by claiming that he had been arrested earlier that day for exposing himself to some Boy Scouts. To be fair, Nathan was only lying to amuse himself, but Simon (who is typically a bad liar anyway) genuinely seemed to think he was being helpful.
In The IT Crowd Jen asks Moss to make up an excuse to get rid of her unwanted suitor, telling him to just say she is busy when he seems nervous about this. Moss then blurts out to the suitor that Jen is dead, causing the whole office to go into mourning, which ends rather abruptly when Jen walks into her own memorial ceremony...
Chuck combines this with Nonverbal Miscommunication. Awesome is trying to explain to Ellie where he's been all day; Chuck tries to help him by miming behind Ellie's back, which results in a story about Awesome decapitating a bear he encountered in the park. Chuck has to step in with a more plausible lie.
Awesome can't really lie to Ellie, but he has no problem lying to other people, even Chuck.
Jeff from Coupling is a terrible liar. His trademark Digging Yourself Deeper monologue is often triggered by telling a stupid, unconvincing and ludicrous lie where the truth would actually have been perfectly acceptable...
Patrick as well, best demonstrated when he accidentally lets slip to Steve that he slept with Jane, then tries to deny it.
Steve: Patrick, I know you. You can't keep a lie like this going!
Patrick: Course I can. Damnit!
Steve: Look, when did this happen?
Patrick: What, the lying or the sleeping with Jane? Damnit!!
Some of the suspects on Lie to Me are painfully terrible liars.
Officer: Anything to declare? Man: Yes... no! No! No! No! Nothing to declare, no, nothing in my suitcase no... Officer: No watches, cameras, radio sets? Man: Oh yes... four watches... no, no, no. No. One... one watch... No, no. Not even one watch. No, no watches at all. No, no watches at all. No... precision watches, no.
This reaches its logical conclusion when the man finally confesses to watch smuggling, only to have the Customs Officer not believe his confession because he's such a terrible liar.
Perhaps the customs agent is just bad at his job; after all, he lets the obvious smuggler (from Zurich, Spain) through, but immediately tosses a collared priest into a side chamber for a strip search.
Spencer from iCarly once tried to take Carly out of school to go to the amusement park. When pressed for an excuse, he said he was taking her to a doctor's appointment. With Dr. Rollercoaster.
Kaamelott: King Léodagan, afflicted with an extreme case of Brutal Honesty, is a bad liar the very rare times he tries it.
Sonny With A Chance: Sonny is a terrible liar. When she lies, her voice usually goes up an octave, and she stammers. The stammering tendency is shared by her boyfriend Chad. Seriously, the cast of So Random must be pretty gullible to fall for the multitude of excuses that Sonny and Chad come up with to hide the fact that they're planning to go on a date. By the time they're all in the limo and have Sonny and Chad cornered, they're either turning a blind eye to get free food, or they are seriously the most gullible people ever to be on television.
J.D.: Since one forty-two yesterday afternoon. His wife didn't want him to do it. She's beautiful, by the way; one blue eye, one green eye. She's from Luxembourg. They're both from Luxembourg. They're Luxem ... bourgian.
J.D.: Outside Mertert, near the German border. They say what they miss most are those lazy summer afternoons on the Mouselle River.
The title character on Miranda. The always single and childless woman once tried to get out of one of her mother's setup by claiming to have a sick daughter.
The characters on Being Human, save for Nina, are pretty bad liars, particularly George. Worse yet, when the group is in desperate need of a good lie he never steps down and lets one of the other characters take care of it; he always comes up with the lie. It is often answered with skepticism.
OWEN: What was it? GEORGE: What? Oh a... pigeon. OWEN: A pigeon? GEORGE: Must have left a window open. OWEN: Have you got rid of it? GEORGE: *beat* I killed it. MITCHELL: You killed it? GEORGE: With a shoe.
In some early episodes of Criminal MindsReid would be reduced to a stammering mess any time he needed to lie. He got better in later seasons.
In an episode of Stark Raving Mad it's revealed that lying makes Henry so nervous that he starts speaking in a Daffy Duck-esque voice. Ian helps him prepare for a situation where he has to lie by drawing on Henry's experience as an actor in college: by rehearsing lies before hand, Henry can then lie convincingly (albeit in a somewhat hammy fashion).
Leslie Knope of Parks and Recreation, honest to a fault, fails miserably at any and all of her rare attempts at subterfuge, in most cases ratting herself out.
On Season 18 of The Amazing Race, the penultimate leg had Flight Time & Big Easy and Zev & Justin find a flight that would put them in Brazil half a day in front of Kisha & Jen and Gary & Mallory, but when Mallory asked the Globetrotters if they had found a better flight, Flight Time's horrible lie all but gave away that they had, and all four teams got on the same flight, resulting in Zev & Justin getting eliminated when they struggled on the leg.
The eponymous character of Merlin deconstructs this trope. He's a bad liar, but he's such a bad liar that most people believe they can already figure out what he's trying to hide, and they assume it's something embarrassing (most of the cast are under the assumption that he's mentally ill) and thus go the other way. So by being a bad liar, he lies perfectly.
Many people in House of Anubis are horrible liars, most notably the main character herself, as well as Mara Jaffray (most notably with her cover story about "endangered hedgehogs" during the first episode.
MythQuest: Cleo, a teenager who is covertly trying to get information about a myth from a family friend and university professor, keeps blurting out details she shouldn't know. Eventually the friend catches on to The Masquerade.
In an episode of Babylon 5, Garibaldi says that Lyta is a terrible liar, which upsets her greatly. It's not seen much in the show, except for one time when she's lying to evil Kosh. He doesn't seem to notice how nervous she is, but Kosh is an arrogantStarfish Alien, so he's probably not good at reading people.
Abby Sciutto from NCIS. It sure doesn't help that all her coworkers are specialist in Perp Sweating and usually know when they're lied to, but even so Abby doesn't like to lie and isn't very good at it.
Rad- Literature! Film! Not Radio! At All!
Arthur in Cabin Pressure goes a funny colour and falls over when he lies. Even when he's sitting down. See, for instance, this dialogue with one Douglas Richardson.
Douglas: Answer this question with a lie. What's your name? Arthur: Arth...nold...man...er...cat...sir...man. Douglas: Arthnoldmanercatsirman... That's an unusual name. Tell me, is it made up? Arthur: Yes, it is. Augh! Douglas: You see, that's the sort of trick question you want to watch for. I'll tell you a secret: the way to lie convincingly is never make something up, just tell a different truth. So, if you have to lie about where you were today, tell them where you really were last week. If you have to give a false name, use a real name you already know. Try again. What's your name? Arthur: Douglas Richardson! Douglas: Better. But not quite perfect.
Needless to say, the advice about telling where you were last week isn't exactly put to good use either.
In Hello Cheeky, John occasionally suffered this trope, usually while trying to conceal something he had just said about his affair with Barry's wife.
John: Matter of fact, one of my best friends is Barry's wife. I mean...I'm in love with a West Indian dwarf.
Tabletop Game examples aren't here at all. Honest.
The "Easy to Read" disadvantage causes this in GURPS. A more potent version (in terms of game mechanics, an even greater penalty to the chance of pulling a lie off) is described as one way to play the Honesty disadvantage (another being, of course, Cannot Tell a Lie).
In Deadlands, the "Lyin' Eyes" disadvantage represents this.
Theatre has positively nothing to do with these examples.
In Animal Crackers, when Spaulding is inquiring about the stolen painting, Ravelli (who helped steal it) suddenly comes up with a dubious theory of what happened:
"Hey Cap, it come to me like a flash. You know what happened to this painting? This painting wasn't stolen. This painting disappear. And do you know what make it disappear? Moths. Moths eat it. Left-handed moths. That's my solution."
These aren't Video Games, these are... uh... Reality Games... yeah.
Iji blatantly stutters when she lies. Fortunately, it matters little when talking to aliens.
Portal: GLaDOS is a habitual liar but her habit of mentioning the truth or creating blatantly obvious fabrications on the fly are readily apparent to the audience.
In the second game, Wheatley isn't exactly good at this either. For instance, trying to "hack" the neurotoxin control system: "Hello Guv'. Neurotoxin inspector, need to shut this place down for a moment. Here's my credentials; shut yourself down. I am totally legit, from the board of neurotoxin, uh, observers, from the United Arab Emirates."
In both games they tell quite a few lies that suggest they have a very low opinion of Chell's intelligence (like trying to convince her to leap into a bottomless pit by saying a sex-starved boy band is down there). Whether she is stupid enough to actually fall for most of them depends on the player.
Knights of the Old Republic: Bastila Shan has a problem with her mouth moving when she thinks, but only when talking to a male player character.
Rune Factory 3's Sofia's lies are made quite obvious as her dialogue will be printed in red to indicate that she actually means the opposite. This is solely for the player's benefit, as the other characters lack the Medium Awareness to see it.
These crop up a lot during the opening levels of L.A. Noire. Granted the game is mostly about reading people, so it's somewhat a Justified Tutorial.
Frank Sawhit in Phoenix Wright: Ace Attorney. Justified, as he's the killer in the first ever case in the series, (and that's not even a spoiler) and thus serves to get the player used to finding contradictions in witness testimonies.
He somehow manages to be even WORSE when he appears in prison in Investigations 2, though.
The Team Fortress 2 "Meet the Medic" video shows that the Medic is a bit of a bad liar: he claims that ribs grow back, and the Heavy looks unconvinced even before the Medic turns to one of his pet doves and loudly whispers, "No zhey don't." It's unclear why he even bothered to claim that ribs grow back, because the Medigun should make it a moot point anyway.
Its funny because ribs DO grow back
Final Fantasy VI's Thamasa is a town full of Bad Liars. Everyone in the town can use magic. They try to hide this fact from the outside world, but they do a terrible job of it once the player characters show up. As with many other examples on this page, this is Played for Laughs, but due to the Crapsack World setting, they once had very good reason to hide their magical abilities from the world, since they were once persecuted for it (they were blamed for the War of the Magi).
These aren't Visual Novels! In fact, Visual Novels don't even exist!
In Little Busters, there is a running gag of Rin starting to say something incriminating, realising half-way through, and finishing the word as something that matches the initial sound but is completely nonsensical. Invariably, she is completely convinced she covered her tracks perfectly. For example, when Kengo asks her a question for which the correct answer is 'Kyousuke', she says "Kyoo-ooooouto is a nice place to live."
Something Positive: Jason kidnapped Anna's supposedly unworthy boyfriend. Anna followed the trail of drag-marks to Jason's apartment and demanded an explanation.
Jason: Well, we've been having a real problem with Canadian Trapdoor Alligators. They scoop you up in a sack and drag you off to their den. It's horrible. Anna: I'm impressed. Somehow I thought you'd be a better liar. Jason: I'm not used to having to lie to a woman I didn't wake up next to.
Zuko is also a pretty bad liar (Azula must have gotten all the lying prowess in the family). In "The Southern Air Temple," Zhao sees right through his clumsy story:
Zhao:(looking at Zuko's ship) That's quite a bit of damage. Zuko: Yes. You wouldn't believe what happened. Uncle! Tell Commander Zhao what happened! Iroh: It was incredible!... what, did we crash or something? Zuko:(looking WAY too innocent) Yes! Right into... an Earth Kingdom ship.
According to the 'Avatar Extras' (Popups that give tidbits of trivia during a set of reruns) Zuko and Iroh have many talents...lying isn't one of them.
Cotton Hill from King of the Hill. To give one example, he tells Peggy numerous war stories in one episode that make him seem like a hero, but simply don't make sense when you look at them all at a whole. (One of them places him at a location hundreds of miles away from another one that, according to him, took place just a couple of days earlier.) Of course, most of the time, Cotton's perception of reality does seem to be shakey at best... Cotton is such a lousy liar that even his son Hank reluctantly admits that he's a liar, as evidenced from this line in an argument he has with his mother:
Hank: Well that means that one of you isn't telling the truth! (Slight pause.) Oh, who am I kidding? I know it's him.
South Park. After being accused of toilet papering someone's house:
Cartman: Okay. Last night, all four of us were at the bowling alley until about 7:30, at which time we noticed Ally Sheedy, the Goth chick from the Breakfast Club, was bowling in the lane next to us, and we asked her for her autograph, but she didn't have a pen, so we followed her out to her car, but on the way we were accosted by five Scientologists who wanted to give us all personality tests, which were administered at the Scientology Center in Denver until 10:45, at which time we accidentally boarded the wrong bus home and ended up in Rancho de Fritas Rojos, south of Castle Rock, and finally got a ride home with a man who was missing his left index finger, named Gary Bushwell, arriving home at 11:46.
Futurama: After accidentally trashing Hermes' office, Bender's excuse is this:
Bender: Uh, it was ghosts! Big ones! And a tornado!
He also once turns off the TV when Leela and Fry come in, claiming that he was watching pornography rather than a cooking show.
Family Guy: Brian's explanation of why he's washing the sheets (that he soiled).
Lois: Those look like our sheets and our quilt. Brian: They are — I'm-I'm washing them in some fabric softener because last night they were so itchy I couldn't sleep which is why I was downstairs when you asked me if everything was okay... You know, 'cause it was so itchy.
Ironically this means he really can't lie — as long as you keep in mind that what he's saying is a lie, swap it around and you've got the truth.
In Dexter's Laboratory, Dee Dee's Imaginary Friend, the Koos-A-La-Goop-A-Goop is a Bad Liar; sort of. He tells Dee Dee that he's the king of the place that he's from, and while he's clearly lying to impress her, she is naive enough to believe him for most of the story. (Of course, Dee Dee isn't all-too bright.)
Mom: Brak, why don't you run along and play with Winston? Brak: Okay, where's he stay at? Franklin: At the bottom of the stairs; you can't miss it. However, if you do, you'll fall through the trapdoor and die. Dad: You have a trapdoor? Franklin: Of course not. Dad: But you just— Franklin: No I didn't.
In an episode of Garfield and Friends, Garfield appears on a Show Within a Show called If You Hear It On TV It Must Be True. Despite the name, he spends the whole show telling rather absurd lies, like "There is no such place as Wyoming."
Phineas from Phineas and Ferb in "The Beak". He scratches his ear, stutters and giggles nervously whenever he lies in that episode, and he lies only to Isabella, for that matter. However, he turns into a Consummate Liar while lying to his mother or Candace.
Sea Captain: Yer... not a good liar, are ya, Dean?
Beezy on Jimmy Two-Shoes. He'll often buckle under pressure and tell people to quit interrogating him, even if they're simply commenting casually to what he is saying.
Applejack from My Little Pony: Friendship Is Magic is defined by her honesty, the Element of Harmony she represents. In the second season premiere, Discord turns her into a liar, but doesn't exactly give her the skills to do it well. Every time she lies, she bites her lip and makes shifty eyes, and when she's actually speaking, she blurts it out in a manner that makes her look like the dumbest pony alive - both in facial expression and content. She gets better at it in the next episode, at which point it's soon resolved. It certainly says a lot about a character's integrity when they have difficulty lying even when being magically compelled to do so.
In "Party of One," the rest of the mane six have to come up with reasons why they can't attend PinkiePie's party. Rainbow Dash and Fluttershy top the list by claiming they're housesitting for "Harry the Bear" who's on a beach vacation... playing beach volleyball/collecting seashells/collecting volleyballs/playing seashells. Rainbow Dash even doodles a watch on her wrist just so she can look at it and "realize" they're late. Of course, since this is Pinkie Pie they're talking to, their excuse is the one excuse she accepts when she starts to put things together.
Applejack doesn't need Discord's help to be a bad liar - also in Party of One, she's perhaps the only one to have a decent, plausible, ready-made excuse for not attending the party (that being that she's busy harvesting apples). It still takes her a good few seconds of gazing over the acres of farmland, buckets of apples, and "um"ing and "ah"ing before she finally manages to blurt something out.
Artemis in Young Justice. She looks away, stutters, and visibly has to fish for ideas when lying. This is made kind of hilarious by the fact that her father and sister are supervillains, and her mother is a reformed one. Which is usually what she's lying about.
An entire episode of Regular Show revolved around Rigby betting Mordecai that he was this, while Mordecai attempted to prove him wrong.
PJ on Goof Troop is very honest, partly because it's just in his nature to want to tell the truth and partly because when he's forced into a situation where he wants to lie he always fails. Comes in several varieties: the thought of lying doesn't even occur to him, he looks for a way to technically not lie, he evades direct questions entirely while making it obvious what the truth is from his tone and body language, his lie sounds like sarcasm despite him wanting someone to be fooled (which, oddly enough, sometimes works), or he comes up with a ridiculous explanation such as "I'm just seeing how hot I can make my knees!" He has been helped by lies before... Max's lies.