A mother of twins was applying for public assistance in Passaic County, N.J., when she made the seemingly uncontroversial claim that one man was responsible for her progeny.

The truth, it turns out, was not so simple.

In an unusual ruling in State Superior Court in Passaic County, Judge Sohail Mohammed found that egg and sperm had colluded to create a medical oddity, according to a report in The New Jersey Law Journal on Thursday. The man who the woman said was the father of her twins was deemed responsible for only one.

The other, the ruling revealed, was conceived during a previously undisclosed tryst that happened within a week of sexual intercourse with the man she claimed was the father.

It was a tangled web of love and biology that gave rise to what The Law Journal called a precedent-setting ruling, one of only a few of such cases across the country. The man originally described as the twins’ father, identified in court documents only as A.S., will now have to pay child support only for the toddler who a DNA test showed was reliably his own.