DNA is scary. There's a case of a woman who had a baby with her husband, but when he got a DNA test, it turned out he wasn't the father. She insisted that he was the father, so they repeated the test several times, but it kept giving the same result.
Later on, he discovered that according to the DNA results, he wasn't the baby's father but its uncle, which didn't make sense because he was an only child.
After more exhaustive tests, they discovered that the husband was actually a chimera, which means he had absorbed his twin in the uterus and carried two sets of DNA. So basically the baby was biologically the child of his twin brother, even though he was the father.