Dehydration is bad. It prevents the kidneys from working, impedes circulation, etc. If you had a map labeled "Dehydration", you would find almost all of it colored "Bad". Unless, of course, the dehydration is preventing something worse from happening, as seems to be the case with respect to the case of the girl trapped in her car for eight days.
Obviously, this is the exception. But I believe that what we call exceptions are the fractal boundaries between one set and another. In this case, the boundary between dehydration being bad and not bad. Where this particular case ended up on our map would be sensitively dependant on a whole host of details such as size and location of the clot, ambient temperature, possibly her blood alcohol level at the time of the crash, and other details that a person with a decent medical education could come up with.