A 34-year-old U.S. man is the first ever reported case of a paternity test fooled by a human “chimera” — someone with extra genes absorbed from a nascent twin lost in early pregnancy.

About one-in-eight single childbirths are thought to have started as multiple pregnancies. Cells from these miscarried siblings are sometimes absorbed in the womb by a surviving twin, but are only rarely discovered by surprises such as the paternity test puzzle.

“Even geneticists are blown away by this,” Barry Starr, a geneticist at Stanford University, told BuzzFeed News.