Science is tricksy.

Actual cases. More info at the link.

Who's Your Daddy? Or Your Other Daddy? Or Your Mommy? - Reason Magazine:

"Case 1: Sean Hollingsworth and Donald Robinson Hollingsworth are legally married in California and are registered as civil union partners in New Jersey. The two husbands arranged for Donald's sister, Angelia Robinson, to serve as a gestational surrogate carrying embryos produced using sperm from Sean Hollingsworth and donor eggs. In October 2006, Ms. Robinson bore twin girls whom she turned over to their two fathers. In March 2007, Ms. Robinson sued for custody alleging that she had been coerced into being a surrogate. A New Jersey court ruled last week that Ms. Robinson, who has no genetic tie to the twins, is their legal mother and can sue for primary custody later this year.

Case 2: A November 17, 2009 New York Times magazine cover article described the case of a man identified as Mike L in Pennsylvania who discovered through genetic testing that the 5-year-old girl he thought was his daughter was in fact the child of his wife’s co-worker Rob. Their marriage dissolved immediately but the cuckolded husband Mike L testified that he agreed to child support when his cheating former wife said that the girl’s genetic father Rob would not support the girl. Two years later, his former wife married Rob, but continues to receive child support for her daughter from her former husband.

Case 3: Lisa Miller and Janet Jenkins were joined in a civil union in Vermont in 2000. In 2002, Miller bore a daughter, Isabella, by means of artificial insemination. The couple broke up in 2003. Now Miller, the biological mother of their child, has become an evangelical Christian, and refuses to allow Jenkins visitation rights with their daughter, claiming that such visits violate her new Christian principles."