The Worsening Curse Mark

The Worsening Curse Mark is a birthmark, scar, subcutaneous pigmentation, magical tattoo, or otherwise permanent/semi-permanent mark on a character that somehow ends up being significant to the plot by a change in it or something unusual that it does; it can't just be static. Disappearance of the mark, the stoppage of its unusual function, or a discovery that the mark was never real is part of this trope. Normal healing of a wound to a scar does not count; a wound that does not heal due to magic or poison may, as long is it does more than just not heal (i.e., bleeds excessively when the inflicter of the wound is near, etc.). Subtrope of Distinguishing Mark. With examples, please include the noteworthy change in the mark.

See also Power Tattoo for marks that grant their bearers some abilities, Marked Change for ones that merely indicate the current status (like Super Mode) and Mark of the Beast for Evil Makeover variety.

Examples of The Worsening Curse Mark include:

Anime and Manga

In Naruto Sasuke's Curse Seal acts as this immediately after being applied. When his will or chakra weaken enough, it spread across his skin like a rash, causing additional weakness and pain. It is later upgraded to allow him to draw on its power without the side effects.
The curse mark on Alan in MAR. If it goes all the way up his neck, he becomes a zombie slave.
In Nanoha Force, the Blue Wings "tattoo" appears somewhere on the body of a person infected with Eclipse and grows larger as the infection progresses.

Film

Ashitaka's curse mark in Princess Mononoke; the worse it gets, the worse the curse is. Now the Trope Codifier as Harry Potter's scar is better known as a Distinguishing Mark and a Birthmark of Destiny.
Jack Sparrow's "Black Spot" in the Pirates of the Caribbean: Dead Man's Chest. It disappears and reappears as his debt to Jones is settled or still needs to be satisfied.

Literature

Harry Potter's scar counts; not only is his scar an indication that he survived the Killing Curse, but it twinges whenever Voldemort is feeling something particularly strong.
The titular character's scar in Robin McKinley's book Sunshine. After Sunshine defeats the vampire who gave it to her, it disappears completely.
In Azure Bonds the eponymous pseudo-tattoo, among other functions, linked Alias to her bosses via their symbols, and there was a reason behind one empty place in their pattern. These vanished one by one when those masters were removed from the scene permanently, and later a mark of the goddess looking after Alias appeared. So when in the third book an old symbol returned...
Artek "the Knife" Ar'talen in Escape from Undermountain was given a magical tattoo after he was "employed" via set-up escape by a noble. It mirrored the sun and moon position in the sky and was set to release a killing spell at the pre-set time—as an incentive to complete his mission and return for removal before the deadline.
In The Dresden Files, a Red Court vampire doesn't fully turn the moment they're infected, but the first time they kill through feeding; they can even use some vampire powers and drink blood to heal and "recharge" without losing their human selves and becoming evil. The Fellowship of St. Giles, a group of these half-vampires, have tattoos that help control their bloodlust. By default, they're invisible, turn black when the half-vampire's using their enhanced abilities and getting hungry, and bright red when those urges are getting out of their control.
Vimes' scar in Thud!, which he gets from tearing his hand open on a rusty nail and gets eerily inflamed whenever Vimes get angry over some injustice. The scar turns out to be the entry point where the Summoning Dark came into his body, and after being burned by burning oil in same location the scar turns into a picture of the Summoning Dark.

Live-Action TV

In an episode of Star Trek: Enterprise, Hoshi Sato is worried that the transporter altered her structural integrity on a molecular level. She cites a mole that moved an inch lower on her face after her transportation as proof. Oh, and she ends up being right.
In Power Rangers Lightspeed Rescue, once Ryan the Titanium Ranger joins the good guys, Diabolico inflicts him with a snake tattoo; every time he morphs it'll climb up his back until it reaches his neck and kills him.

Video Games

In Final Fantasy XIII, a l'Cie's brand indicates how much time he has to complete hisFocus before becoming a Cie'th. Fang's brand is white and blurry, indicating that she's not on a timer. This happens to the brands on the rest of the cast at the end, and the brands disappear entirely after defeating the final boss.

Western Animation

Aang's arrow tattoos in Avatar: The Last Airbender. They glow when he has entered the Avatar State. The arrows also indicate a Marked Change.
Invoked in the short-lived children's series Sagwa the Chinese Siamese Cat. In one episode, Sheegwa, Sagwa's younger sister, is discovered to have a star-shaped marking on her fur that is "the mark of the princess", indicating that Sheegwa is actually a princess. Unfortunately, the mark turns out to be dirt.

Real Life

Skin cancer. If one of your Beauty Marks turns color, gets bigger, or otherwise starts looking wonky, see a dermatologist pronto!
And, of course, if any sort of wound doesn't heal the way it's supposed to (i.e., scab over with minimal scarring), it's probably infected.