An ABO typing discrepancy can cause a delay in selection of products that are safe to transfuse to the patient. This can lead to a depletion of the supply of universal donor red cells and plasma. Knowing the possible causes of ABO discrepancies helps to develop algorithms and standard procedures to resolve discrepancies quickly. A deeper understanding of the process can be gained by working through example cases. Special consideration is given to discrepancies due to patients changing blood type after hematopoietic stem cell transplantation, where various strategies can be employed for reporting of the patient's blood type and selecting products for transfusion.