Hemoglobin A1c (HbA1c) is a very common measure utilized to diagnose diabetes and to monitor the level of glycemic control during the course of management. Despite the high utility of HbA1c, it has some limitations. Physiological conditions that affect the lifespan of red blood cells (RBCs) can falsely elevate or lower HbA1c results. In this case report, we present a case of a patient who was found to have hereditary spherocytosis (HS) after developing nephrotic range proteinuria. The patient had diabetes that was previously thought to be well controlled, but his HS was masking his poor glycemic control. This case highlights the importance of understanding the limitations of the HbA1c in managing patients with diabetes.