accidental incest; when one has sex with a girl/guy, only to later find out that person is a long-lost brother, sister, cousin, etc. occurs most often in white-trash families in the deep south
Joe Bob: "Cletus had sex with Brandy, only to discover she was his sister, and they had been separated when they were both babies."
Billy: "HAHA! He had accidincest! What a creep."