I mentioned in my superfat post that a superfat that is too high can lead to rancidity in the finished soap. But how to we tell if a soap has "gone bad?" The biggest indicator is something that soap makers call Dreaded Orange Spots, or DOS for short.
DOS can be any variation of orange, including not really orange at all. It can appear all over the surface of the soap, or only in one localized section. There may be one spot, or many. There is no real way to tell how DOS will appear, if it does occur. I have heard that soap with DOS is still safe to use, however I'm not sure that I would be able to bring myself to do that. None of the soaps I have for sale will ever have DOS at the time of sale, and should not occur after purchase as long as it is stored properly.
So what exactly causes DOS? I'm not sure there is a specific way to be able to answer this, but I've seen many different theories. Some say that oils that have gone rancid many contribute to a finished soap developing DOS. However, others say that they have made soap from rancid oils and had no problems. My personal opinion on the DOS theory is that it is caused when there is too much unreacted oils in a soap. Because most soap makers cannot determine precisely the amount of leftover oils in their soaps, this can affect the superfat level. I typically superfat my soaps at 7-8%, and I have never had a problem with DOS.
For more information on DOS, Anne-Marie has put together a great blog post, complete with pictures, and just this subject. Check it out!