Blood transfusion is not 100% risk-free. The most common reactions are mild allergic or febrile reactions and are not life-threatening.
Additional transfusion risks and reactions:
Severe allergic reaction
Respiratory distress due to fluid overload (Transfusion-Associated Circulatory Overload), or injury to the lungs (Transfusion-Related Acute Lung Injury)
Bacterial contamination
Fever, chills, rash
Hemolytic transfusion reaction (an immune reaction where antibodies lead to the destruction of transfused red blood cells)
Mistransfusion (human error leading to the transfusion of the wrong product)