Acute myocardial infarction (AMI) diagnosis in patients with pre-existing left bundle branch block (LBBB) can be difficult. Undiagnosed or delayed diagnosis of AMI in these patients can put them at risk of having shock, mechanical complications, and death. We present a case of 77-year-old Caucasian male with a known LBBB and coronary artery bypass surgery for coronary artery disease who presented to the emergency department with a chief complaint of chest pain and shortness of breath. The patient had recurrent chest pain despite using aspirin, nitroglycerine, and morphine. An electrocardiogram (ECG) showed a new notch in the upslope of the R wave in leads I, AVL that indicated a positive Chapman's sign. Troponin levels were initially normal, but serial troponin showed elevated enzyme giving evidence of acute coronary syndrome (ACS). The patient was started on heparin drip and underwent subsequent coronary catheterization. Physicians should be aware of Chapman's sign on ECG in patients presenting with chest pain who have baseline LBBB as it might represent myocardial ischemia and warrant emergent treatment for ACS.