Technically a Smile

"What? That was a smile! I smiled!"

—Calvin

Related to the Cheshire Cat Grin, Slasher Smile, The Un-Smile and Psychotic Smirk, Technically a Smile is when the narration (or narrator, in the case of first-person) notes that the mere fact of baring one's teeth doesn't necessarily make a "smile", in the sense of an expression designed to convey friendliness, happiness or harmlessness.

Humans are one of the few mammals to bare their teeth as anything other than a threat; this trope comes into play during those times when a "smile" conveys something more... animalistic. Look for the phrase "didn't reach his/her eyes" to appear.

Obviously, this is far more common in print media. In comics, film and TV, the expression doesn't have to be described because the audience can see it right there.

Examples of Technically a Smile include:

Anime and Manga

Averted in Wolf's Rain; even in human disguise, the wolves never smile with their teeth. Neither does Cheza, but it might just be because she's pretty subdued in the first place. Either way, Cher and Hubb (the two actual humans), seem to instinctively pick up on the fact that smiling + teeth = bad around wolves. It's not like they have much to smile about once everyone starts dropping like flies.
Played with in Mahou Sensei Negima. When Big Bad Fate attempts an Evil Laugh, it seriously freaks out Asuna, because "it doesn't touch his eyes".
Kiyo in Zatch Bell frequently smiles when he's not at all happy with a situation; it's more of an indication of altogether irritation and/or embarrassment, and is usually accompanied with Mind Control Eyes. Other characters, like Folgore and the mangaka himself, are almost always drawn with that kind of smile, though.
In Bleach, Kensei trying to comfort Hisagi in the Turn Back The Pendulum arc.
Balalaika (pictured above) does this occasionally. It is on those occasions that the audience is reminded that, despite her affability, she's still a ruthless mass murderer.
It is noted about Sesshoumaru from Inuyasha that if he ever smiles, it is generally not a good thing.
The Millennium Earl from D.Gray-man always smiles, which is creepy enough. When he gets really, really angry, his smile gets very frightening.
For a visual example of the smile "not reaching someone's eyes", check out November 11's picture on the Phenotype Stereotype page. Although only a few Contractors have Mind Control Eyes (the protagonist being one example), most Contractors (as seen by November 11) have fairly dull eyes. Thus, it's noticeable for all Contractors, the protagonist included, that their eyes will acquire a warmer look if they are genuinely smiling, but otherwise, chances are they are faking it/displaying Dissonant Serenity.
This Neon Genesis Evangelion parody, courtesy of Shortpacked!.
- Which is also a Call Back to Amber's "smile" in this strip.
- There is also this other parody of the same moment. Notice the AT (Absolute Terror) Field.
Sunako Nakahara from The Wallflower, when she is asked to smile, smiles like this.
Izaya Orihara of Durarara!!. It is noted occasionally in the graphic novel that his smile 'doesn't reach his eyes.'
- When Akane first meets Shizuo, she gives a smile that's compared to the sort a child would make while stomping on ants. Then she attempts to kill him.
When Ren of Skip Beat! gets angry, he doesn't shout or frown. Instead, he smiles. He smiles so hard that the Bishie Sparkle can blind onlookers. Kyoko and Yashiro find this to be incredibly terrifying.

Comic Books

In an issue of G.I. Joe Special Missions, the Joes go to a rural house to rescue a family from terrorists. They see one guy who always has a crazy-looking grin on his face. They initially assume that he is one of the terrorists. Turns out he's one of the hostages. He has a rictus, a facial spasm that causes a permanent teeth-baring grin.

Fan Works

"Ms. Aoyama" from Desperately Seeking Ranma. Descriptions of her smile always indicate that it is anything but pleasant and reassuring; at least one character thinks of it solely as "that terrifying thing she did with her mouth". It frequently has an effect equal to or beyond that of a Slasher Smile without looking anything like one.

Film

Wednesday in the film Addams Family Values makes a pseudo-smile after an attempted brainwashing by being subjected to Disney movies. It's incredibly creepy.

Amanda: She's scaring me!

In the Director's Cut of Terminator 2: Judgement Day, John Connor tries to teach the titular killer robot to smile. The results are horrifying enough that John abandons the attempt.
When Hancock is beginning to reform, he gives an imitation of a smile at a press conference. It looked more like a snarl.
Toothless in How to Train Your Dragon attempts to smile when Hiccup smiles at him. It comes off as more endearing than scary though, because Toothless is Ugly Cute.
Michael Keaton's Batman smiles a couple of times. It's easily the scariest expression he's got.

Literature

Seems to occur about once a chapter throughout The Wheel of Time series, with the phrase "didn't reach his/her eyes" attached about 80% of the time. In the latest book, one of the Wise Ones makes a comment about Aes Sedai and smiles to show she's joking. The narration notes that she only succeeds in baring her teeth.
The Discworld books have this from time to time, usually with werewolves and vampires, but sometimes with Vetinari, and once even with Vimes.

"Vimes' smile was as funny as the one that moves very fast towards drowning sailors. And has a fin on top."

- Rincewind does it at least once too—in his case, it's a rictus grin of sheer terror.
- And Granny Weatherwax, who has the "corners of her mouth turn up slightly when something bad is about to happen to someone she thinks deserves it".
- The Librarian (a three hundred pound orang-utang) smiles a lot. Or at least, his lips turn up showing lots and lots of big yellow teeth.
A staple of Larry Niven's Known Space books. Kzin are predators, born and bred. So when you can see all of their teeth, this is not a friendly expression.
In American Gods, Wednesday has a habit of grinning without a hint of amusement or kindness, and it doesn't take long before Shadow is reminded of chimpanzees baring their teeth to show hostility.
In The Dresden Files book White Night, Marcone is described as making a facial expression where the corners of his mouth turn up and he shows his teeth, but doesn't smile at all.
- Harry himself is also prone to a rather shark-like Slasher Smile when he's about to set someone on fire...
Anne McCaffrey's Acorna series eventually has the titular Petting Zoo Unicorn reunite with her species. There's a hiccup in the road because she, having been raised by humans, will show big toothy grins which are threat displays to the unicorn people. It's really the only notable instance of culture shock.
From Charles Stross's The Concrete Jungle:

"[Angleton] smiles, and despite all the horrible sights I've seen today so far, I hope most of all that I never see it again before the day I die."

In Isaac Asimov's The Caves of Steel, human Lije Baley asks humanoid robot Daneel Olivaw, "Can you smile?" Daneel can, technically, but it doesn't reach his eyes and Baley finds it more unnerving than reassuring.
Javert from Les Misérables. "Would you like my hat?"
Morrachane, an awful woman in one of the Circle of Magic books, has such an unnatural-looking smile that it takes the main character a second to figure out what her face is doing.
One of the few signs that Ford Prefect is a Human Alien.

Perhaps it was that he smiled slightly too broadly and gave people the unnerving impression that he was about to go for their neck.

X Wing Series: Wedge Antilles sometimes wears a smile that's pure predator. As he gets older it becomes even more so - in Legacy of the Force he once flashes a smile at someone "like he was a rancor, and they were made of meat."
- A good deal of the works in Star Wars Expanded Universe, particularly that which took place during her political career, depict some variant of the sentence "Leia grinned, showing teeth," giving the notion that the gesture is not particularly friendly.
In Johannes Cabal the Necromancer, the title character is described at one point as a "tallish, blond pale man who only ever seemed to deploy a smile as an offensive weapon."
Quite commonplace in the Honor Harrington universe, with Exact Words eventually showing up when things get really nasty. See the quotes page.
The Abh are described as having elevated this to an art form in Crest of the Stars. When it was animated the version Lafiel gave was... pretty intimidating.
In Roald Dahl's The Witches the first witch the protagonist encounters is described thusly:

"When most people smile their lips go side-to-side. This lady's went up-and-down, showing lots of her teeth and gums."

Live-Action TV

Michael in Burn Notice often laughs and smiles, but rarely conveys actual warmth, having great difficulty showing emotion thanks to his abusive childhood and training as a spy. Notably, his fake smiles when dealing with his family are much more obvious than the ones he uses to deal with criminals, due to their somewhat strained relationship. Some of his antagonists have similar mannerisms. Take a good look at Victor's smiles sometime; he looks like he wants to bite out your throat.
The Dick Van Dyke Show: "Honey, your teeth are showing, but your lips are tense."
Hilary Briss is always doing this on The League of Gentlemen. The bottom half of his face is split in an enormous Slasher Smile, but his eyes remain the same. Actor Mark Gatiss must have spent a lot of time in front of the mirror practicing, because it's creepy.
Sheldon of the Big Bang Theory cannot smile (or laugh) to save his life. As Leonard put it: "We're here to congratulate Koothrapali, not kill Batman!"
Cromartie's attempts to smile in The Sarah Connor Chronicles are not convincing.

Newspaper Comics

Calvin and Hobbes: Calvin does a grotesquely awful/hilarious one for a photograph.

Tabletop Games

Inverted in Werewolf: The Apocalypse. One of the things Lupus Garou, wolves who turn into humans, have to get used to when dealing with people is the difference between a smile and a threatening grimace, when both are likely to show teeth.

Video Games

In Final Fantasy X, Kimahri attempts to smile once. Tidus' response is "Sad."
- Though Tidus is the poster child for "Technically a Laugh".
  - I honestly didn't think that was a smile up until people said it; I thought that Kimahri thought Tidus was making fun of him by telling him to smile and was bearing his teeth. Seriously.
In Fire Emblem 7, Raven attempts to smile during one of his support conversations. It hurts his face to do so.
Civilization IV: In the event that Montezuma is furious with you (a very common occurrence unless you happen to follow the same religion he does), he'll spend his diplomatic meetings baring a grin that could crack a sink, teeth grinding the whole way until he loses patience and screams in frustration.
Baraka from Mortal Kombat always seems like the cheerful type, but that most likely comes from having More Teeth Than the Osmond Family.
This was implied in Sonic Unleashed where one of the NPCs asked Sonic why he doesn't smile in his Werehog form and wants him to do it. Sonic does so (albeit offscreen) and the NPC was startled by the discovery, regretting that he asked.
There are about three instances where Atsuki Saijo smiles in Lux-Pain. The first is when he fights with Mika over the strawberry shortcake. The second one is forced on him by Akira in archive 1. The third one, which is the most disturbing of them all is Atsuki loses it when Edward calls him a hypocrite. Not only does he smile, but he laughs similar to Light...

Webcomics

Inverted in Freefall, where Florence (an anthropomorphic wolf) has to remember not to show her fangs when she smiles in case she scares people off.
- Although she has discovered that many arguments can be averted by calmly stating her case and then flashing a big, toothy smile.
- Her smile looks forced because it is. The same as with the Werewolf example above.
Mordecai of Lackadaisy Cats shows how it's done.
- Rocky is also a master of this as shown here
Hannelone from Questionable Content cannot smile
- Nor can Faye.
Basic Instructions, twice. Had its own devoted strip here, and sent up with "what is a smile but a grimace of happiness?"
Luca in The Meek. Just to clarify, in Pasori culture, toothy smiling does not connote good humor.

Western Animation

In Disney's Beauty and the Beast, Lumiere attempts to coach the Beast in order to help him win Belle over, "Give her a dashing, debonair smile. Come, come, show me the smile." The Beast tries, but the result mostly involves baring a mouthful of very sharp fangs and is not remotely dashing or debonair.
The Joker's gas victims. They die with a smile on their faces. After laughing uncontrollably. In pain.
- To be precise, it's the uncontrolled laughter that kills them.
  - It's worth noting that even the producers and writers thought it was probably more horrifying than simply killing someone with a gunshot.
In The Pagemaster, Horror "smiles" to show he's actually very friendly. Unfortunately, he has bad teeth, thin lips, and an ugly face to boot, so it comes off as frightening/hideous anyway.

Fantasy: "Aw, look, he's smiling!"
Richard: "That's a smile?"

Octus gives his best go at a full grin on Sym-Bionic Titan. It's quite off, but he really does mean it.
How the Grinch Stole Christmas: The Grinch had a wonderful, AWFUL idea.
- Hey, he was happy about it!
In one episode of Hey Arnold! Arnold is forced to pair up with Willie the Jolly Olly Man, an ice-cream man who happens to be an intense Child-Hater and a Talkative Loon for Career Day. To help him keep his job, Arnold tries to make him smile to appear friendlier to his young customers, but his grimacing teeth-baring grin just sends the kids running off in terror.
The prehistoric episode of SpongeBob SquarePants has Patar give what might be a smile, but just looks disturbing.

Real Life

We humans are the only ones who turned smiling into an expression of joy. If a chimpanzee (and possibly most apes) grins at you, it is more likely an expression of fear or aggression than friendliness. It's because when you grin, you show your teeth, as in "bare your weapons". Grinning is an expression of fear that became a show of submission in social primates. It's thought that's why it became a friendly gesture in humans - it's advertising that you don't want any trouble.
- A friendly chimpanzee smile, if it shows any teeth at all, only shows the lower ones. Even then, humans might find it to look threatening (at least someone has noted this about friendly bonobos smiling), which would be a Subversion.
There are several accounts of dog owners getting growled at, nipped, or even bitten by their frightened dogs after smiling.
- This is because in "dog language", baring your teeth and/or snarling - the motions of which can be simulated by an enthusiastic smile - is considered to be a threat. In their minds, they're simply defending themselves after you've threatened them.
Dalmatians are the only dog breed that can approximate a smile, usually done so if they know they're in trouble or are just very happy. It looks like a snarl, but they do not show their back teeth, and they're usually wagging their tails.
- Some Jack Russell terriers can do it too.
- A mutt, half beagle and half schnauzer, may be capable of doing this.
- Large amount of mongrels is capable of kind of smile.
Most Swedes smile without showing their teeth, and doing so is seen as a little intimidating. Apparently, "only crazy people or Americans smile a lot with their teeth."
According to legend, victims of cyanide poisoning will die with a horrifying grin on their faces, due to muscle spasms.
- It's called rictus and it can happen to any victim of facial muscle spasms. It may be caused by paralysis as well.
Some people just can't stop grinning when they are stressed. Needless to say, this causes misunderstandings.