Achey Scars

Scar tissue is assorted connective tissue the body produces in an attempt to close up an injury. While sturdy, it tends to lose the original tissue function, including touch and feel. In real life, pain in scars is usually caused by sensitivity in the skin around the scar rather than in it.

In fiction, this tissue has an interesting ability to ache or throb when someone's old nemesis comes by, especially if the injury was caused by said villain. (Such significant injuries are of course also permanent). Achey Scars can also refer to scars that were acquired a lifetime ago that still make the bearer gasp or cry out when touched by either a love interest or a particularly perverse big bad.

See also I'm Having Soul Pains and Wound That Will Not Heal.

Anime

 * Yukito was born with a scar in AIR that suddenly relives the time that his past life received the same scar, causing Yukito to collapse to the floor.
 * In Ai Yori Aoshi, Kaoru's scars (the result of years of child abuse dealed by his grandfather) have a tendency to ache whenever he's reminded of his family (whether during flashbacks, nightmares, anxiety attacks, etc.). It's a major point in the plot's resolution when they finally stop hurting.
 * Also, they tend to hurt less when in presence of his Victorious Childhood Friend Aoi.
 * In One Piece,.
 * Whitebeard's also complained of aches from scars he received from Shanks' old captain.
 * Duval's scar on his back used to hurt whenever he thought about Sanji. It was forgotten about when he underwent a Heel Face Turn.
 * Shortly before the Time Skip, Luffy got a scar on his chest from admiral Akainu. First time Akainu is mentioned after the timeskip, Luffy takes a hand to his scar and squints one of his eyes as if in pain, implying that this trope also has affected him.
 * Turn a Gundam goes so far as to apply this to the eponymous Gundam's arch-rival, the Turn X, which sports an X-shaped scar across its chest that glows in the presence of the Turn A.
 * Getter Robo has Hayato Jin, whose scars are normally invisible but suddenly flare up all over his body when tense.
 * Eiji Date from Hajime no Ippo has a scar on his nose bridge, which hurts sometimes when it's cold or he's tense.
 * It stops throbbing
 * Cain in Godchild and Count Cain has whip scars across his back, inflicted by his father, which ache whenever his father's evil organisation is doing something bad, i.e., near constantly.
 * Most of Xanxus' scars in Katekyo Hitman Reborn remain hidden until he gets really really angry.
 * Eureka gains a lot of scars in Eureka Seven after certain episode, which stick around for most of the series.
 * In Saiyuki Hakkai's old scar sometimes aches when it rains.
 * The Brands of Sacrifice act as this for Guts and Casca of Berserk when demons draw near, doubling as Wounds That Will Not Heal as well.
 * Dilandau has one inflicted by Van in Escaflowne.
 * Dilandau has one inflicted by Van in Escaflowne.

Literature

 * In Steven Erikson's Malazan Book of the Fallen, Tok the Younger bears a scar in place of a lost eye. The scar itches in the presence of magic and during pivotal scenes whose outcome further the novels' plot(s).
 * In the Inheritance Cycle Eragon has a scar on his back through most of Eldest, left by the Shade Durza that continues to cause debilitating spasms of pain throughout the book. Presumably it was some magic in the wizard's sword. Fortunately for Eragon, it goes away when towards the end of the book.
 * Harry Potter, although there is an active magical component to his scar.
 * In addition to the lightning bolt scar he got as an infant, which was explicitly magical, he acquires another one in book five after the Tyrant Takes the Helm. Professor Umbridge forces him to write lines with a magical quill that carves what he wrote into the back of his hand. This left the lines permanently scarred into his hand, and they tingle every time he thinks of Umbridge. Whether this is psychological, or if it's just another perk of the quill is unknown.
 * The Lord of the Rings, where Frodo's injury from a cursed weapon aches when the One Ring's power is active, specifically during the Council scene when Gandalf reads the inscription. In the book, he falls horribly ill on every October 6 thereafter, the date he acquired the injury.
 * I think I read somewhere that Gandalf thinks Frodo's pain in mainly psychological. How many times have you been stabbed with a ring-wrath's blade? Huh? HUH?
 * The blade was cursed, and it's not surprising that the pain continued. If Gandalf's quote about the Shadow (read: evil) returning is to be believed, Frodo might actually have a connection with any malevolence left in the world.
 * He also has pains that trouble him on the anniversary of losing his finger and being stung by Shelob (at least until he leaves for the Grey Havens).
 * Shelob was something of an Eldritch Abomination (to say nothing of her mother), and it's not impossible that her venom was still in Frodo's system. After all, we never saw him presented with any type of antidote. So perhaps the psychological symptoms brought on the physiological ones.
 * Gurney Halleck, from Frank Herbert's Dune series, has a deep-red scar down one side of his face which causes residual pain at times of great stress.
 * The prequels by Brian Herbert and Kevin J Anderson attribute this to him being struck with a Harkonnen inkvine whip, inkvine being a poisonous dye that leaves permanent painful scars.
 * Captain Ahab in Moby Dick can apparently feel in his missing leg whether Moby Dick is nearby, although his insanity makes it hard to tell if this is actually true.
 * Ahab also sports a (presumably) body-length scar obtained either from the whale or the incident with the Spaniard. It functions as a lesser form of the leg, contributing to his madness.
 * Carnival of the Deepgate Codex series has a multitude of scars, which constrict and even start bleeding when she's in a state of mental trauma.
 * Kavi, in the Farsala Trilogy, had his hand badly scarred by a deghan as a child, and it still pains him as a young man.
 * In The Wheel of Time series, main protagonist Rand al'Thor gets not one, but two. The first is taken in a fight with the Dark One's Dragon. It's not able to heal, but at least the Aes Sedai keep it from spreading. He then gets a second on right next to it from a Darkfriend weilding a cursed weapon. The healers aren't even able to contain the second wound, so how do they keep it from killing him? Why, release the bonds on the first wound so that the two corruptions fight each other rather than spread. This keeps Rand alive, but in constant pain.
 * In The Wheel of Time series, main protagonist Rand al'Thor gets not one, but two. The first is taken in a fight with the Dark One's Dragon. It's not able to heal, but at least the Aes Sedai keep it from spreading. He then gets a second on right next to it from a Darkfriend weilding a cursed weapon. The healers aren't even able to contain the second wound, so how do they keep it from killing him? Why, release the bonds on the first wound so that the two corruptions fight each other rather than spread. This keeps Rand alive, but in constant pain.

Video Games

 * In the Street Fighter Alpha series, any time Ryu and Sagat meet up, Sagat clutches the massive scar Ryu gave him on his chest... which glows.
 * Not only that, in Alpha 3 you can make his scar glow on command and make his next Tiger Blow a lot stronger as a result.
 * The scar on Shiki's chest tends to hurt whenever he sees traffic accidents or large quantities of blood...but also seems to have a distressing tendency to bleed in really stressful situations.
 * Possibly because
 * To be fair, it's not a normal wound and in fact apparently disappears entirely after SHIKI dies.
 * Reiji in Namco X Capcom and Endless Frontier gets a pain in the scar over his temple whenever Saya, the fox-woman who gave it to him, is near.
 * Fang had one in Final Fantasy XIII because her l'Cie scar has been warped and could not change since she was thawed out of her crystal.
 * In Dragon Age II, the white scars all over Fenris' body cause him some fairly intense pain. Justified, as they're more like tattoos made out of lyrium than anything else, which has a well-documented range of nasty side effects.

Western Animation
"Steve: He's thinking about me!"
 * In Transformers Animated, Ratchet finds his old war wound is acting up again. Turns out the Bounty Hunter who made it came to Earth to capture Optimus Prime.
 * In a season 5 episode of American Dad, one of Roger's alter-egos stabs Steve for not getting an anniversary present for their parents. He and Hailey flee to Argentina when his scar starts to hurt.

Real Life

 * This type of pain is usually of psychological origins. For example, if you have a scar on your arm from a chainsaw-related accident, there's a great chance that if you even see a chainsaw, or hear it working, that scar will hurt a bit. See also: phantom pain.
 * This can occur if a scar develops into a keloid (WARNING: disturbing photos.).
 * Adhesions can result as a junction between scar and normal tissue. These hurt.
 * Nerve damage can result in this too
 * Or when the scar tissue is putting pressure on a nerve bundle.
 * Scars (or, rather, the tissue around said scars) also tend to be painful well after the fact if they are in a particularly sensitive part of the body, eg the abdominal area.
 * While it doesn't really ache as such, knocking a large scar against something like a table or door can make it feel odd and can make the surrounding area ache.