One day, out of the blue, every microscopic injury or tear occurring within your muscles will repair itself by replacing the affected tissue with bone. Over time this process will either result in joints locking into place, limiting mobility and the ability to perform rudimentary tasks, or you'll be recruited to the X-Men in a plot-twist that reveals you're Colossus' bastard son.
Good or bad is really a matter of perspective here.
The most common early presentation of this disease is being born with oversized toes. Seriously. It's what helped popularize the the old axiom "You know what they say about guys with big toes, right? They probably have fibrodysplasia ossificans progressive. But they might have a big dick too."
I'm Scared. How Scared Should I Be?
Fibrodysplasia ossificans progressiva is a very rare disorder, with only several hundred cases reported worldwide. That said, if you've won the ossification lottery, you're kind of fucked.
We already mentioned your body remodeling into the Tin Man, probably prompting you to beg passersby to give you a sweet hit off an oil can. What you may not have considered is that your jaw is likewise as prone to this as anything. You could just as easily cease to eat or talk over time. The real rub is that none of this is treatable by surgery because it turns out scalpels somehow injure your body as well.
Man, How Much Does Your Body Hate You?
= As much as Satan hates bands using rock music for Christianity.