I was diagnosed, with some difficulty, back in 1995, when I was 15 years old, with Localized Scleroderma, or Morphea. The disease sites an overproduction of collagen as a major symptom, and as a result, I make extra scar tissue.
At least half of my right arm is covered in darker toned, mottled skin, and that skin itself is actually thinner than normal, so that if I bump my arm against something, it hurts like crazy, but not immediately -- it usually takes about 15 seconds for the discomfort to really rock in, and then I'm pretty unpleased for about 2 minutes, and then it's all better. If it bruises, I can't tell, since I'm colorblind.
The marks and scars have been spreading a bit on the right hand, and it looks like my middle finger should be wholly scarred within the next year or so, like my ring finger is already. An interesting process to watch, to say the least.