I am fairly sure this has happened many times in the list of people in many researcher's Ahnentafel. (Ahnentafel is a word commonly used in genealogy although it probably confuses most newcomers. Ahnentafel is a German word that literally translates as "ancestor table". It is a list of all known ancestors of an individual and includes the full name of each ancestor as well as dates and places of birth, marriage, and death whenever possible. It also has a strict numbering scheme.) Irish Couple Discover They Are Siblings This sounds like it is straight from a supermarket tabloid but, apparently, this story is accurate. A couple who now have a young son were shocked to find that they are half-brother and sister. The couple, who grew up in separate towns about 100 miles apart, were adults before they met. DNA testing last month revealed they share the same father. This might mess up the family's pedigree chart. Statistically, this was bound to happen sooner or later and probably has happened before. The odds are similar to that of winning the lottery. You can read more at http://www.thaindian.com/newsportal/world-news/young-irish-couple-devastated-to-find-they-are-siblings_100372805.html