It happened to my wife. She was O+ throughout her childhood according to her mother and as confirmed by her medical records, etc, (her uncle is in immunologist and her cousin is a tech in a hematology clinic). It wasn't until some time around the age of 18, when she applied for a driver's license and had her blood type checked again, that she realized her type was different. It came back as B+. She's had it checked several times since then to confirm. Still B+.
I had never heard of it before this before I met her. She's not had any transfusions or transplants of any kind.
