Israel Pickholtz shows one unique way that a false positive might occur. This emphasizes why an approach based on the statistically expected amount of shared DNA can never give a certain answer. I've also been surprised with negative DNA results in cases where lots of circumstantial evidence seems to show a definite relationship. I suspect that a lot of that may be due to orphans gratefully adopting the surnames of the family that took them in. After all, with the wars and epidemics that were prevalent in Europe, there must have been many cases of informal adoption.
