(repost from G+)
I was thinking about Patrick Stuart's 'Scars' and I couldn't help but think they were lacking one category. I tried to make it up, then two more appeared. Probably very unbalanced mechanically.
Complicated [Mechanical] Things
Traps, engines, golems, all kinds of constructs, probably guns if you don't want to bother with gunshot scars. Golems can have many forms and not all mechanical things are purely mechanical, as sometimes a complex magic is involved as well - maybe this is why scars from those things are always serrated wild zigzaging patterns that run in parallels lines and branch like on an abstract motherboard; there is some weird logic to this pattern that can be felt instinctively but never fully understood. Scars have metal line just in the middle of each line, thin metal thread gleaming within a thicker line, looks like zipper teeth. Draw thick line with a marker, than a thin line within it with silver colour. Such scars never move, they are firmly embedded and seem to grow just so slightly with each passing year.
From a purely mechanical thing the thicker line is a dull grey flesh, harder on touch than the rest of the skin, always slightly colder than the rest of the skin. For more magical devices the scar tissue is changing colours, occasionally into impossible ones, kind of pretty and not so shameful to show in thieves guild as a scar from lowly mundane traps. Golems and other constructs leave bit of themselves around thin metal lines - it might ooze tar, stone dust or rust, sprout tiny glass ornaments, be hardened to a bone.
People say that mechanical things are just tools of the whoever built them but you know it isn't true. They have a will of their own, a purpose of their own, they want to hurt you. They remember you. You know they seek you out to finish what they started.
Exalted trap senses against particular type of traps or constructs, perhaps. DM might tell you what trap whispers (it is always something malicious and inhuman but might betray its danger) or if a door is saddled with such parasite (a door might complain). Sense inactive golems when they camouflaged as statues, for example; be able to hurt them slightly better without special weapons, be able to predict their moves. In stressed conditions can live off on fuel if the mechanical thing runs on it (gunpowder, oil, coal, uranium) for some time before being fatally poisoned from eating indigestible toxic material; counts as a ration a day for three days, mildly poisoned/weakened on second day, then die if continues to eat this stuff past third day.
Persistent sense of paranoia about machines maybe confirmed by anything as complex as a crossbow malfunctioning or breaking twice as often or twice as severely in their hands.
(I think it grew slightly out of hand, almost like a small class by itself)
Surviving one's execution is definitely a luck bordering on supernatural or a supernatural bordering on luck. Not all executions are survivable and most aren't (guillotine isn't, decapitation isn't, quartering definitely isn't) but sometimes rope snaps before the damage is done, or bullet glanced the heart instead of piercing it, or injection isn't working, or you dug yourself out of grave on a brink of suffocation, or a governor's pardon is delivered just a moment after the switch is thrown, despite all expectations.
Execution should be the real and be carried off sincerely, no hopes left. False executions or pretended ones or bravely escaped ones or those interrupted by friends with a clever plan don't count. Scars look fresh however much time passed (the appearance doesn't impede in a system-mechanical way except maybe in social situations) and always are slightly unnatural - traces of hanging rope form what looks like letters and are bright red, suffocation leaves face pale with shade of cyan and mildly inflexible, throat in blue hue that deepens when you near the graveyard or deep water and so on. People with Second Sight and ghosts always see such scars through clothes, any disguises and armour.
Supposed to be dead but isn't. Mindless undead don't usually attack first, vampires extract no nourishment from blood. Blood is tinged grey, very slightly. How uncouth.
Supposed to be dead but isn't. Can speak to graves, learn who is buried there, their regret, their pride, their joy, their pain. Mostly useless tidbits. Mostly.
Supposed to be dead but isn't. Supernatural luck with dying - if reduced to dying/dead state, just before losing consciousness, can transfer the killing blow to any active ally in vicinity, once per moon or on any equinox and solstice. The ally is getting dying state (or put down to 0/1 hp if DM is generous) regardless how healthy they were before while for scar-bearer this wound simply didn't happen.
Supposed to be dead but isn't. All fortune telling cannot read your fate. There is somebody who was sent from Dead Realms for you soul and is still looking for it, still is to claim it - they have their deadbooks to keep, they don't like paradoxes, things out of balance. All received crits are one step more severe (or twice as much, however your play it). There is probably a grim reaper on your trail.
Supposed to be dead but isn't. Devils love paradoxes, paradoxes are useful tools for them to have. In any social encounter with devils, they will take an extra effort to tempt or charm or enslave you to their service.
Supposed to be dead but isn't. People tend to forget your face until you, directly or indirectly, did something harmful to them or people they care about. Then they remember your face all too well.
Probably not much use in typical DnD, and are hard to write about because so many different variations of guns and their wounds are possible.
In a sense the gun is a mediator between a creature and a mechanical thing. Triggered by person's intention and hand but attacking on their own, like a trap.
Gunshot scars probably look quite mundane, but framed by darkened tendrils of flesh, like an eclipsed sun corona. They tend to slowly drift toward the heart with years.
You always know the name of whoever shot you even if you never saw them. Even if they changed their name you still know it, you know all of their names.
Ignition that made the bullet fly is gone but your flesh is still seared from the inside by it, part of this flame lives on. Sense any fire as small as bonfire/campfire within a mile or two and an approximate direction to it.
Bullets live fast lives. Initially they are cocoons of death that can wait patiently for years and then go through chrysalis, unfold their wings, reach maturity and die in less than a second. You can move fast even if paralyzed, blind, drunk or otherwise held, impeded or slowed down. Each such action ages you one year of life.
If the bullet went through there is a tunnel of void in a wake of its passing. Like some lonely, invisible, trembling harp string is here, however you sew the wound later - and if you tell a story about yourself that has at least three kernels of truth (and the situation is not rage-madness-homicidal) people will stop to listen as long as story goes and they will probably believe it. You can always tell if somebody is truly in love with you.
If bullet is stuck inside, it is your secondary mini-heart of unfeeling steel. People have very difficult time manipulating your emotions, be it fear, rage, love or sympathy, by magical or mundane means. Things might recognize you as one of their own - half chances of malfunction or severity of it for anything as complicated as crossbow. You have difficulties with empathizing and see less colours that aren't grey. You cannot raise your voice to shout or yell loudly about anything anymore either (if you are barbarian, you are very quiet barbarian). You can speak to small simple non-magical metal things (keys, nails, rings, etc) and ask them simple questions or for a simple favour.
If bullet didn't go through and neither it remained in the wound, you have a hungry hole in your flesh. You eat and drink twice as much as a normal person, but are able sleep with your eyes open and can never be surprised by beings with heartbeat within gunshot distance of the weapon that left the initial wound. In scrutiny you can eat gunpowder instead of food for three days or less, just like a rations. Your vomit and urine are mildly flammable after such nourishment and became progressively more flammable and combustive up to alchemical fire severety after three days of gunpowder 'food'. Once per moon you can eat another bullet, negating all damage from it; it is devoured without a trace.