I hope I'm not too late!!
My mom is a labor and delivery nurse. One time she had a patient whose parents were present during her stay at the hospital. The staff there have a white board with some general information about the patients like blood type, time spent in the wing, and like how far along. This information is nameless for HIPAA reasons and instead has a patients number.
So this patient happened to be the only one in the wing at the time. Her father was walking back from the cafeteria at one point, and noticed the board. He stopped one of the nurses and asked if the blood type for the patient on the board was correct (he knew his daughter was the only patient there). It was B+. The nurse confirmed it was correct.

Turns out he knew his and his wife's blood type. He was O+ and his wife was A-. This combination will never yield a B+ baby. He brings his wife out and just points at the board and this look of shock washes over her.

TL:DR Father of patient in labor finds out his wife cuckolded him 25 years later by knowing his blood type punnit squares.