We present the case of a 24-year-old male, who received a minor ABO-incompatible allogeneic hematopoietic stem cell transplant (HSCT, blood group O+ ⟶ A+) from an HLA-matched unrelated female donor, as consolidation therapy for relapsed precursor-B-cell acute lymphoblastic leukemia. The donor had a known history of Hashimoto's thyroiditis before HSCT. At day +10 posttransplant, the patient developed severe hemolysis, which required emergent red blood cell exchange. Additionally, about a year posttransplant, he had circulating antithyroglobulin antibodies, decreased free-T4 (fT4) and increased serum thyroid-stimulating hormone (TSH). The potential causes of the posttransplant hemolytic episode and hypothyroidism are discussed. While the hemolysis was worsened by the transfusion of A red blood cells (RBCs) in the context of passenger lymphocyte syndrome, the thyroid dysfunction might be explained by an autoimmune disease transferred from the donor. The case highlights the possibility of several non-relapse-related complications of HSCT occurring in the same patient. It is critical that such adverse outcomes are distinguished from classical graft-versus-host disease (GVHD) for adequate recipient counseling, posttransplant screening, and prompt treatment.