Biological, factitious, and technical reasons have previously been reported for real or apparent changes in blood type. In two cases, an apparent change in blood type resulted from a previous impersonation of the patient by a different person.
(JAMA 241:159-160, 1979)
Littler ER, Silverman EM. Anomalous Blood Typing From Impersonation. JAMA. 1979;241(2):159–160. doi:10.1001/jama.1979.03290280039024