In 2006, a mother named Lydia Fairchild went to apply for public assistance in Washington state. By law, herself and her two children had to be tested to prove that they were related. When the DNA test results came in, no one could prepare themselves for the shocking results.
Although Fairchild had given birth to both children, her DNA and theirs showed that they weren't related. Worse yet, the welfare system in Washington could take this as a charge of welfare fraud and even talked about taking her children away.
Yet there were the birth certificates, listing her as the mother. She could also call upon her obstetrician, Dr. Leonard Dreisbach, to testify in court, as he had been present when she had given birth to the children. But what's more, Fairchild was also pregnant at the time with a third child - and the court insisted on having a state-appointed representative at the birth to witness it personally, and draw DNA samples right there at the hospital. The results from this DNA test came in two weeks later - and again, she showed as not having been the mother of the third child!
As the court date for welfare fraud drew near, Fairchild's attorney happened by luck across an article about chimerism in the New England Journal of Medicine. In chimerism, an organism can be born with two distinct groups of cells in the body, forming two completely different sets of DNA. This can occur when two separate sets of fertilized ovum can fuse together in the womb - in other words, genetically speaking, Lydia Fairchild was actually two people walking around in one body!
After a hectic series of tests of more people in Fairchild's family tree, it was determined that this was the case. The case was resolved, Fairchild got her family assistance, and everybody learned something new.
Lydia Fairchild on YouTube (NOTE: this TV show claims four children; the original Wikipedia article I link to says two children with one on the way. I'm going with confusion stemming from possibly having a fourth child by the time this episode was produced.) :