I've never had it happen to me personally, but there have been several snakes that this has happened to over the years. It's not genetic and some have theorized that it's the result of a bacterial infection but I don't think that's ever been proven. Generally people have reported that it started with an odd rather waxy looking defecation followed by a shed that appeared to be thicker then normal. I'm not sure if anyone has ever figured out how it happens, but it appears to strip away the top layer of pigment and gradually replaces it with melanine. It certainly is odd and I've never heard of it happening twice to the same person.
Didnt RDR do a vid on youtube about it? I seem to remember seeing one on it.