Secret Stab Wound

Such was the severity of the situation, that no one noticed that Wolverine's wounds from the previous battle had not yet healed. And such is the calibre of the man known as Wolverine, that he pressed on, saying nothing at all to the others.

—Professor Xavier, X-Men Next Dimension

A subtrope of the Game-Breaking Injury and/or Bullet Holes and Revelations. When a character hides that they've received a GBI, and in some cases may be Secretly Dying. The key is that it's not necessarily revealed by a fight scene, and that they hide it from their friends as well - differentiating it from Mortal Wound Reveal. Often accompanied by the injured party stating that "it was nothing", "I'm fine", or after the seriousness is discovered, "I Can Still Fight".

Only a Flesh Wound if the character survives, and the Truth in Television aversion of the Instant Death Bullet when the injury is fatal. In the appropriate setting, this may lead to the sufferer becoming a Zombie Infectee. If the injury is discovered posthumously, it can fall into Time-Delayed Death. If someone is particularly good at hiding the wound, their friends might not notice until they collapse, or maybe not even until after that.

Compare and contrast Major Injury Underreaction.

Examples of Secret Stab Wound include:

Anime and Manga

Otonashi in Angel Beats!. After the train he was on is derailed, and the survivors are stuck underground, Otonashi spends the next week helping everyone, despite the fact that he was badly injured. He kept this a secret so that no one would freak out. He dies seeing everyone getting rescued. Luckily he was able to donate his organs.
Tsume in Wolf's Rain, probably. Its possible he just didn't realize he was that badly hurt until his adrenalin wore off, since he seemed rather surprised to see a pool of his own blood. In any case, (he doesn't get better).
Lina from Slayers gets a stab injury early on, and doesn't tell Gourry about it. She casts a gradual healing spell on herself and relaxes in the sun, waiting for the wound to heal, and Gourry misunderstands that she's just being lazy. Until he grabs her and finds out that she was hurt.
In Great Teacher Onizuka, Onizuka is shot while rescuing a girl from kidnappers, but still turns up to an exam that he must take in order to keep his job. His injury is not revealed until the girl wakes up in hospital and asks why Onizuka isn't there too, and a pool of blood is seen under Onizuka's desk.
Ban in GetBackers did a sort of Diving Save (without the diving) for Ginji who was about to be stabbed by Akabane's Sword. Afterwards, he lead Ginji to believe that it was an illusion caused by his Evil Eye. He didn't reveal it till later.
While he's not fooling another character, November 11's death in Darker than Black fools the audience in a similar way as the GTO example- in both cases, the audience knows that they successfully took down a room full of enemies and knows they were injured, but doesn't realize how badly until you see a pool of blood flowing out of them.
- Similarly, Huang had a bullet wound in his stomach throughout the entire Grand Finale. Blowing himself up with most of his pursuers was just a last hurrah.
In Robotech, Roy goes on a date with Claudia after coming back from a battle and collapses from injuries. He apparently knew he was wounded and refused medical treatment, perhaps because he suspected the doctors couldn't save him, and wanted to see Claudia one last time.
Trigun: Vash the Stampede duels Brilliant Dynamites Neon to stop a runaway train (a huge runaway train ...). After his first clever maneuver, however, Vash accidentally rips concealed and previously bandaged wounds open and can't keep up his normally calm demeanor, falling down. Neon, fortunately, is a man of his word and helps stop the train.
In Tenchi Universe, Ryoko suffers one of these while fighting Kagato. The injury lingers through the penultimate episode, being further aggravated when Ryoko is attacked while disabling Jurai's planetary defenses. She holds out just long enough to pilot Ryu-Ohki to the landing docks of the Jurai Royal Palace and deliver Tenchi before seemingly succumbing to her injuries.
Date Masamune in the Sengoku Basara anime gets shot, but doesn't reveal the injury until miles from the battlefield where he'd been riding his horse unconscious for quite a way. Leads to a Shirtless Scene and two I Can Still Fight moments. A glorious injury all around.
Gunslinger Girl. Hilshire gets shot in the gut during an assassination he carried out himself instead of leaving it to his cyborg. Triela notices something's wrong on his return to the hotel room, and is so upset (she's conditioned to protect him at the cost of her own life) that she runs out on Hilshire despite his efforts to play down the injury.
All Might from My Hero Academia was dealt a Game-Breaking Injury by All For One pre-canon and keeps it a secret from the public up until he burns out his last embers of One For All on live TV during the Kamino Incident.
Used several times in Madlax, usually for some variety of Mind Screw. Carrossea Doon manifests a bleeding wound after looking through the door of truth, but it's not a recent injury, but part of the revelation that he was actually Dead All Along, having taken a bullet for Margaret as a boy twelve years earlier; his apparent growth to adulthood was an effect of Margaret's refusal to remember the events during which it had happened while she tapped into the Reality Warper power loosed that night. Eleanor is clearly shot in the back by a soldier while searching for Margaret in the jungles of Gazth-Sonika, then hours later appears seemingly none the worse save for obvious exhaustion -- only to collapse and die from a wound that wasn't visible until that moment when a mind-controlled Margaret rejects her.

Comic Books

Strongbow in Elf Quest TOS (he gets better).

Film

In The Adventures of Huckleberry Finn Huck gets shot in the back while he and Jim are fleeing a lynch mob, and he tries to hide it so that Jim will escape.
Nicholas is stabbed at the end of The Reckoning when Lord de Guise gives him a hug, and he hides that fact in order to follow de Guise out of the Church so that the crowd can see what he did. This fact is also hidden from the audience until that point.
Maximus being stabbed by Commodus could also fall into this, but it was already mentioned in Game-Breaking Injury.
Shaun's mum in Shaun of the Dead hides the fact that she's been mortally bitten by a zombie and will shortly become one, so that Shaun wouldn't worry.
LJ also hides a zombie bite in Resident Evil Extinction.
Ladyhawke: Phillipe hides from Navarre the fact that he mauled him while he was in wolf form.
Horatio Hornblower: In the Mutiny/Retribution story, Archie tries to hide from Horatio that he was shot in the fight onboard with the Spaniards. He doesn't do a very good job of it though.
Doctor Smith in the Lost in Space movie hides that he was bitten by a spider and is transforming into one.
Andre the thug hid the fact that his wife was zombie bitten in the 2004 Dawn of the Dead movie.
In Baz Luhrmann's William Shakespeares Romeo and Juliet, Mercutio plays off his mortal stab wound as not being that serious.
At the end of Children of Men, Theo receives a secret gunshot wound. He survives long enough to help Key escape, before succumbing to his wound.
Also in Blood Diamond, where Danny Archer receives a fatal gunshot wound to the lungs. He survives long enough to help Solomon and Dia escape before peacefully dying from his wounds.
A rare self-inflicted example forms the climax of Black Swan.

Literature

The Lord of the Rings. After Frodo is pinned against a wall in Moria by an orc chieftain's spear, he says he can walk (and does so). After the Fellowship escapes, Frodo feels more pain with every step and is gasping for breath but doesn't tell anyone. Eventually Legolas notices them falling behind the others and Aragorn treats Frodo's injury. (Frodo was wearing a mail-shirt, so the injury was a horrible blow with bruising and the rings driven into the flesh, but all in all not as bad as could have been.)
There was this Forgotten Realms book that this troper read; in one section, it described a dwarven legend who got his belly cut open in a battle, but quickly tucked his innards into his shirt to prevent any of his men from noticing, and continued fighting.
Pulled off very darkly in Terry Pratchett's Monstrous Regiment: The soldiers coming home from the war have their red coats buttoned up tight, even in the summer's heat... because they don't have any bandages.
In the Doctor Who Eighth Doctor Adventures novel Fear Itself, the Doctor does this after getting more or less harpooned in the stomach while on a space station because he doesn't trust any of the medical staff around to help him. He buttons up his frock coat and Fitz performs some very basic first aid on him when they're back in their cabin. Good thing he can heal. Too bad he just has to go heroically carrying someone out of a fire before he gets better—his wound opens up again.
In John Moore's The Unhandsome Prince, the title character wins a tournament with an enchanted sword ... except that when he struck the winning blow, his opponent's dagger slipped beneath his breastplate and gravely wounded him. (The judges didn't see it.) Since the winner had bet on himself (at 100-1 odds), he couldn't reveal his injury or he'd lose the tournament (and his winnings, which he needed to keep his family from going bankrupt).
In Catch-22 when Yossarian's plane is hit with flak, his crewmate Snowden has a minor injury and appears to be in shock. We see snippets of Yossarian comforting Snowden all through the novel, thinking Snowden's injuries are minor - but Yossarian finally discovers "Snowden's secret" - an untreatably massive trauma that means Yossarian can only hold Snowden's hand and watch him die. This could almost be a trope namer, as it's a pretty famous example.
In James Swallow's Black Tide, when Vetch takes a bullet for Rafen, Rafen calls him a fool, and he chides him as disrespectful and reveals that the wound he took in the fight before that one had been poisoned.
Older Than Print: Sir Gareth of Arthurian legend jousted with a Black Knight. Both knights were thrown from their horses on impact, but while Gareth was unharmed, the Black Knight's armor was penetrated and he abruptly fell dead during the ensuing swordfight.

Live Action TV

In the 2006 BBC version of Robin Hood, Marian is stabbed by Guy while in her Night Watchman disguise, and doesn't reveal this to Robin, until she collapses in the woods after their escape.
Sara in Los Hombres De Paco hides the fact that she was shot in the stomach in order to go through with her wedding to Lucas. He only discovers this when she collapses after the post-ceremonial kiss.
Charles Westmoreland / D.B. Cooper from Prison Break. He got stabbed in a fight with Bellick and hid it until he broke down during the great escape. He died, but not before leaving his wish in Michael's hands.
In an episode of Farscape D'Argo managed to outrun the bad guys with Crichton before collapsing from a gunshot to the back.
In the second season finale of Leverage, Nate gets shot in the side by Kadjik and hides it from the crew until they're safely away and he's turned himself in. It's probably not life-threatening, but...
In the epilogue scene of Choujin Sentai Jetman, Gai, a.k.a. Black Condor is stabbed and hides it from his friends until he actually dies of his wounds.
In the Horatio Hornblower movie Retribution, Archie Kennedy is shot during battle and doesn't reveal it to Horatio until the battle is over. It was a fatal injury, leading Archie to take sole blame for the mutiny to save Horatio's life and career.

Music

"Romeo is Bleeding" by Tom Waits from the album Blue Valentine
A variation, in the spirit of the captain with the red jacket and brown pants: Dispatch's "The General" features a general who apparently doesn't want his troops to worry about getting hurt like him:

He grew a beard as soon as he could
To cover the scars on his face
And always urged his men on

Theater

Cyrano De Bergerac is an older example of this. When Cyrano is dying of a head wound, he keeps it from Roxane until while telling her about court gossip, until he comments that "Cyrano de Bergerac was foully murdered".
In a (possibly) still older example, Mercutio's death scene from Romeo and Juliet is often played this way.

Video Games

Godot, under his mask in the 3rd Ace Attorney.
- Also in Ace Attorney, fourth case of the first game, von Karma's secret bullet wound was so secret he never got the bullet taken out. While he was recuperating he didn't attend any trials.
  - Which eventually led to his downfall
- And who could forget Quercus Alba who concealed a stab wound until he needed it for his alibi.
Fire Emblem has a couple of these in more recent installments:
- Seth receives one of these at the very beginning of the game, though it doesn't affect his battle performance. He recovers well enough, though Eirika remains quite worried. Their Support Conversations talk about this injury as well.
- Greil in Fire Emblem 9 (Path of Radiance). Unique in that it was self inflicted.
In X-Men Next Dimension, described in the page quote, Wolverine had received several battle wounds, as could likely be expected. The problem is, one wound taken was a Power Nullifier, stopping his healing and letting him take damage from future fights, which he was about to charge into, as the situation mandated it.

Real Life

After a gun misfire, an airman had the firing pin sewn inside his leg because the surgery would have required him to go back home otherwise.
I heard there was some battle in which one side took over 100% casualties, meaning that a significant portion of the force was injured, missing, or captured, returned to fighting, and became casualties again. Might have been Iwo Jima or Fallujah... someplace with less than pleasant weather, and a rather lengthy battle, more than a few days...
- John Keegan, in his history of WWI, mentions that New Zealand had 8,566 soldiers at Gallipoli. They recorded 14,720 casualties for just this reason.
- The 3rd Armored Division in WWII's European theater had 16,122 casualties out of a 16,000 strong division. Division Commander Major General Maurice Rose was killed in action, one of 11 KIA US Generals in the war. There's a fantastic book called Death Traps: Survival of an American Armored Division in World War II written by a mechanic. He excoriates the M4 Sherman as a piece of crap both offensively and defensively. His story about hosing out the prior, deceased crew before manning the patched-up machine with a new tank crew certainly lends weight to his claim.
There's a joke to this effect: a captain faces down his old adversary in ship-to-ship combat. Before the battle begins, he turns to his bosun and says, "Fetch me my red shirt." The battle commences and his men manage to destroy the enemy ship with no casualties. The first mate asks the captain why he needed the red shirt. He says, "I always wear my red shirt into battle, so that if I'm injured, my men won't know and they'll continue fighting." Suddenly the lookout shouts that there are ten more ships on the horizon, bristling with cannons. The captain says, "Fetch me my brown pants!"
Admiral Yi Sun-Shin died this way, suffering a mortal injury due a chance shot during the Battle of Noryang. Witnessed only by three people, his final orders were to press the attack home and to hide his death from the crew and fleet, to avoid ruining morale at a critical moment. His final victory was thus won posthumously.
Perhaps even more famously than the above example, the death of Horatio Nelson at Trafalgar.
Occasionally, paramedics will have to chase down someone who has taken a severe stab wound but insists that they're fine... right until they fall over from blood loss. Because a stab can cause a lot of bleeding or organ damage without affecting very many pain nerves, it's quite possible to underestimate just how badly you've been injured.
- Adrenalin also decreases the pain response, so if the injury was sustained in a fight or during a risky accident where adrenalin would be released, they might genuinely not feel even the pain that would be there.