In January of this year, I published a brief article that referred the reader to a longer article in the BBC web site that claimed historians had found blood in an old gourd that contained the blood of French King Louis XVI. The blood reportedly had been obtained when the king's head was severed by the guillotine. The historians were so confident of their find that they even offered DNA tests to compare your DNA with that of the blood from the supposed French King. These "tests" supposedly proved whether or not you are related to French royalty.
There is only one problem with that story: further research now shows the DNA taken from the 18th century gourd does not match that of DNA samples taken from French King Louis XVI's documented living descendants of today. In other words, some scientists claim that the blood in the gourd could not possibly be that of King Louis XVI.
HOWEVER, there is some dispute as to the true descendants of the French kings.
There is a possibility that the living descendants are not really descended from King Louis XVI, despite the claims of the old documents. Therefore, nobody can say for sure right now if the blood in the gourd is or is not that of Louis XVI.
You can read more in an article by Elizabeth Pennisi in Science Magazine at http://goo.gl/EoUNf1.
DNA may be a valuable tool for genealogy but it still depends upon accurate record keeping over the centuries to provide accuracy.