Usually happens in an action scene - bullets and blades flying every which way - chaos and mayhem dominate. At either a lull in the fighting, or the final victory of one side (usually the heroes), we see one of the characters standing fairly still, looking pale. Then something about the scene changes. Either the camera pans to reveal a gaping hole that really shouldn't be there, or essential parts of the character drop off. They don't have to die instantly, but the wound has to be lethal, and revealed.
The heroic version tends to be used in works where Anyone Can Die, and played for tragedy. The villainous version tends to be either a) the loved one of the Big Bad, so they go berserk - which usually humanises colder villains, or b) a major villain has been killed, most often The Dragon or the Big Bad, bringing that arc to a close, sometimes unexpectedly early. Mooks are too small fry to waste serious stage directions on.
May arrive after a Gory Discretion Shot, as a sort of Subversion though this is by no means necessary. More often there's no clue given that anyone on the team that thinks they won is hurt beforehand. May very well be used in an example of Not Quite Saved Enough.
See also Gun Struggle and Bullet Holes and Revelations. Some forms of Robotic Reveal subvert this trope - the robot, of course, does not die from their 'mortal' wound.
Compare Almost Dead Guy, Secret Stab Wound, After-Action Healing Drama, Zombie Infectee (where the injured party deliberately hides that they've received a wound, rather than the author/director hiding it from the audience), and Game-Breaking Injury (where it's specifically the Hero who receives a significant - though not necessarily mortal - wound).
Non-lethal examples will usually go under either Secret Stab Wound or Game-Breaking Injury, or if they really don't fit either of those tropes for some reason, go here as a Playing With of some kind, invoking either No One Could Survive That, or No One Should Survive That.
This is a Death Trope so Unmarked Spoilers follow. You have been warned!
In Otonashi's flashback sequence in Angel Beats!, Otonashi has just managed to rally and negotiate peace throughout the survivors of the subway cave-in accident and walked off to get a break when he pulls his jacket back, revealing the blood spreading under his skin from the terrible internal injury that eventually kills him.
Following a lengthy fight, Solomon in Blood+ staggers into a dark alley and rips open a small cut on his clothing. Turns out a small wound with Saya's blood was enough to make his body slowly crystallize and crumble.
Played With in Fullmetal Alchemist. Ed fights Kimblee only to be taken by surprise when Kimblee reveals he has a Philosophers Stone, which he uses to collapse the mineshaft serving as their battleground. After the collapse, Ed wakes up on the floor of the mineshaft, suitably pissed off. When he tries to get up, blood pools around him, and the camera pulls back revealing that he's been impaled with a metal bar from the collapsed mine. He actually manages to live—by using his own soul as energy to boost his medical alchemy, saving his life now at the expense of a shortened lifespan.
Gundam 00 has a particularly cruel example, where Lichty's Heroic Sacrifice for and reveal of his cyborg nature to Christina Sierra is then followed by the reveal that she'd been pierced by a chunk of shrapnel and he hadn't noticed before he died in her arms. She too dies shortly afterwards, still clutching his body.
About halfway through the first Macross anime, Fokker's plane is hit during a mission. He lands and proceeds to continue his plans for the evening. The fact that he was wounded is only revealed when the maintenance crew finds blood on his pilot seat and the extent of it when he silently passes away on his girlfriend's couch.
As a Shout-Out, Macross Frontier set up the exact same thing with Ozma getting hit in a battle and going to Ranka's concert fully knowing he's gonna die but him passing out from blood loss alerted his subordinates that something's wrong, saving his life.
Trigun arguably subverts this, since it's obvious in a meta sense that Nicholas Wolfwood isn't long for this world following his confrontation with Church, but since Vash is too deep into an Heroic BSOD to notice, the direction doesn't make it explicit until their actual death scene.
Bleach has a not-so-villainous example with Ulquiorra. He's able to get back up and stop Ichigo's Super-Powered Evil Side after being on the receiving end of a Curb-Stomp Battle, so none of the heroes realize how injured he really is until his remaining wing explodes into ashnote a sign that his Healing Factor wasn't working. Then again, his thoughts also reveal that his organs are not coming back.
Tsume from Wolf's Rain got seriously injured in a battle. His pose suggested he was deliberately hiding it, but then he looked like he was surprised to see his side ripped open and a huge pool of blood.
Naruto loves to invert this trope. A ninja gets filled with shuriken, stabbed, or nailed with a high-powered Jutsu. A few reaction shots... and then the predictable reveal that the ninja A. was a Clone, B. used a Substitution Jutsu, or C. the situation was not what it seemed in another way (Genjutsu, Transformation Jutsu, a last-second counter-move mitigated the hit somehow, they have some sort of regenerative or protective power...)
A tank example in Girls und Panzer. During the Final Battle, the Hetzer manages to execute a plan to destroy the enemy's Maus tank, but shortly afterward, it shuts down from the damage it took; the tanks have a system in which if they're hit with sufficient force, or take enough damage, a flag rises and they're out of the game.
Mulan is revealed to be injured after the initial defeat of the Big Bad but not mortally so. Although it could have been had she not received medical attention.
V for Vendetta has an example, where initially it appears that V has superhuman powers by surviving a barrage of gunfire. After dispatching his foes while they frantically try to reload, he removes a home-made metal plate - riddled with bullet holes and covered in blood - from under his shirt, drops it on the ground, and staggers off. (Before the Mortal Wound Reveal, there's a subtle indication: his raspy, pained voice as he delivers his Kirk Summation.)
Resident Evil - The Laser Hallway scene in the first film has two of these. As the first laser goes by and shears off one commando's fingers, the leader turns to The Medic asking her to help, but she doesn't respond. They look again, and the camera shows a paper-thin red line appearing through her neck before her head falls off. A few seconds later, after a laser grid passes through the leader, he falls apart into chunks of flesh.
Tristan and Isolde: Happens to Tristan near the beginning of the film. His wound initially appears not that serious, but just as we're informed that the weapon was poisoned he collapses, apparently dead. Being the protagonist, he gets better, but his friends aren't Genre Savvy enough to know that.
In the trilogy's second film, the heroes appear to have escaped through a door just in time to avoid a hail of gunfire from the army of Smiths chasing them. then The Keymaker turns around to reveal that his torso is riddled with bullet holes.
During the final battle in Gangs of New York, after the duel is interrupted by cannon fire and the smoke clears, Bill the Butcher and Amsterdam sit up. It's revealed that Bill has a shrapnel wound in his gut.
In the Sherlock Holmes parody The Very Strange Case of the End of Civilisation As We Know It, John Cleese as Holmes is shot in the stomach several times with what were supposed to be blanks as part of a clever gambit. When Watson fesses up that he'd forgotten the switch, Holmes surveys the growing bloodstain on his shirt, and irritably scolds Watson for his bungling.
Holmes: It'll be nice to be dead... get a few things done. (Dies)
The Ninth Configuration ends with Colonel Kane saving Capt. Cutshaw from a biker gang at a bar. Following the brawl, Cutshaw carries the battered Kane back to the asylum wrapped in a blanket, but it isn't until he's alone that Kane lets the blood-stained knife drop to the floor and begins composing a farewell letter. In the film's original ending (and the story on which it was based), Kane palmed the knife and committed suicide, but the studio insisted at the last minute on changing a line of dialogue to imply that he was actually stabbed in the chaos. (Some argue this was actually an improvement, since Kane's intended lesson - that humans are capable of selflessness - would be broken by suicide.)
One of the interpretations of the ending to Black Swan.
In The Hunt for Red October, Vasily Borodin is standing in front of the weapons console when the saboteur starts shooting. He slumps down and Ramius rips open the outer part of his uniform, revealing that he had been shot.
Happens to Graham in Paul when he gets shotgunned in the chest. He even fails to realise for a moment that he's been shot badly enough to destroy his heart.
In The Godfather Part III, at the end there is an attempt on Michael's life. Shown are the two gunshots, Michael being hit in the shoulder, and the gunman being killed. As the commotion dies down, the camera cuts to Mary, Michael's daughter. He looks at her, and she looks down to reveal that she has been shot in the chest. She then dies seconds later.
Happens in The Quick and the Dead, when two characters face off in a gunfight. Both fire, and one appears fine until he looks down at his shadow and sees light coming through a bullet hole in his chest.
In the second Young Guns movie, Chavez gets wounded at some point during the fight with Sheriff Garrett and his deputies, but we're not really aware of the wound until he starts acting hurt when they reach safety. Later still, Billy finally catches up to them and Chavez shows him what turns out to be a pretty horrifying stomach wound. Shortly thereafter Chavez limps out and does off to die by himself.
In Blood Diamond Danny has a final confrontation with his merc friends and doesn't even notice that he's wounded until after the fight is over, he goes to pick something up and pulls up short in pain. It takes him awhile to die, so he manages to first outrun the other mercs for awhile and then is carried by Solomon when he can't run anymore, but he finally insists on Solomon going out without him.
In Underworld, as Viktor is kneeling over Michael and about to kill him, Selene picks up Viktor's sword from where he'd dropped it earlier and slices through him with it. At first, it seems like she whiffed badly, as he actually jumps up completely intact and draws daggers from his sleeves in response - but then Selene holds up the blood-covered sword, which is followed by half of Viktor's head falling off.
In Skyfall, it is revealed in the middle of the climax that Judi Dench's M is bleeding to death from what seemed to be a near miss, a wound that proves fatal.
In the finale of Scream, Sidney stabs Billy in the shoulder with an umbrella, but doesn't cause any apparent wounds. Later, when the two are grappling, he has a gaping hole there, big enough for Sidney to stick her finger in.
Occurs in the opening of Scream 2 when the killer, disguised with Phil's jacket, stabs Maureen in the chest.
Happens in Face/Off with Castor Troy's criminal associate Dietrich during Castor's attack on his former hideout. We see him jump into the line of fire just in time to intercept a bullet Castor had intended for his girlfriend Sasha. However, we don't see that it's a mortal wound to the side of the neck until Archer shows up.
The Remake of Land of the Dead has Michael show his bite wound to Ana and tell her he can't get on the boat with them, despite her insistence she can heal him. The last you see of Michael is him putting a gun to his head as the boat leaves.
In Joseph Heller's Catch-22, Yossarian attempts to tend to the small wound of a crew member who was hit by flak on a bombing mission. But when he loosens the crew member's jumpsuit, he finds a huge exit wound, and can do nothing but attempt to comfort the dying man. The non-chronological order of the narrative makes it a bit hard to tell, but this is the experience that pushes him into him into becoming the paranoid, cowardly Cloud Cuckoo Lander / Only Sane Man that defines his character for most of the novel.
In the post apocalyptic book, Last Refuge, two characters have left the last refuge to travel across a wasteland representing the Hindu Book of the Dead. After tangling with a cannibal and needing to get away as quickly and as far as possible, the wounded person hides his wound so that his friend can get away without slowing them down by trying to give medical aid.
In Parellity, we get a nice long description of the gang successfully defeating the last Corsair buggy before we learn that Doug's been shot.
Happens to Mels in the Doctor Who episode "Let's Kill Hitler" when one of Hitler's stray bullets hits her in the stomach. This leads to the reveal that she is River and causes her to regenerate into her.
In The West Wing at the end of Season One/beginning of Season Two, the president and his staff are the targets of shooters. After the shots, Toby attempts to find Josh and comes across him sitting against a wall. At first it seems that Josh is fine, but as Toby walks closer he sees that Josh is holding his stomach over a bullet wound. Being a major character, Josh does survive. This trope is also played fairly straight with the President, another bullet victim noticed after the fact (he also lives).
In the second series of Horatio Hornblower, this happens to Archie Kennedy. Although the wounding is shown (he is shot at and sits down abruptly), it isn't until the aftermath that Horatio rips his coat open and sees that he's been shot in the stomach. However, it takes him quite a long time for the wound to kill him; the wound was mortal largely because of the state of medicine in early 1800's Europe.
Played with in Dominic Deegan with a wound that would have been lethal if the characters weren't already in Hell.
The Order of the Stick has a particularly extreme example; after Miko destroys the Gate in Azure City, causing a massive explosion, she is seen lying on the ground from the chest up, speaking with Soon Kim's ghost. After her highly ironic last words ("I can live with that"), we see that she has been ripped in half at the waist.
Mulan: The eponymous heroine discovers she is seriously injured. The mood dramatically changes as she and her friends realise that she's wounded, and changes again when, in the process of treating her wounds, everyone realizes she's a woman.
Played with in the Looney Tunes short A Corny Concerto. Porky Pig and his hunting dog are hunting Bugs Bunny, and after a chipmunk gets ahold of Porky's rifle and shoots it, the three of them stand to the camera with their hands over their chest. Porky and his dog both cautiously check under their hands, and realize they're fine. Bugs peeks under his hand, goes green and collapses. He's faking, of course.
In Transformers Prime, Dreadwing is shot by Megatron and we get to see a close-up of the gaping hole in his chest.