My sister-in-law found out her brother was a half brother. OOPS, mom had an affair.
Most if not all of the Oops* here on the west coast, that we know about, have been from Mom’s having secret affairs or flings. Then, Oops, along came Ancestor.com..
*That we know of.
I remember a song from several decades ago, set in a Caribbean island. A young man falls in love with one young woman after another, only to have his father tell him that she’s is sister “but your mama don’t know.” Finally he goes to his mother in desperation about his predicament and she tells him it’s OK because his daddy isn’t really his daddy “but your daddy don’t know.”