Insulin pen devices have revolutionized the self-management of diabetes. Prefilled insulin pens are preferred by healthcare providers and patients because of ease-of-use, dosage flexibility, disposability, and widespread availability. However, like any mechanical device, these pens have the potential for mechanical malfunction. We are present a case in which glycemic control deteriorated because of a malfunction of an insulin pen. The possibility of insulin pen malfunction needs to be considered in addition to other factors in patients who present with acute deterioration of glycemic control.