We describe two patients with posterior myocardial infarction (PMI) with unusually atypical clinical presentations and cardiac enzymatic profiles, but with the abrupt development of complete AV block in patient 1, and Mobitz II second-degree AV block with paroxysmal phases of higher degrees of AV block in patient 2, and mitral regurgitation leading to symptomatic pulmonary congestion. Also, both patients had complete right bundle-branch block (RBBB) [old in patient 1, new in patient 2], the pattern of which was altered due to the associated PMI. The alteration included tall R waves involving the early part of the QRS complex, and tall T waves, both noted in the right precordial leads. The mechanism of these ECG modulations of the RBBB pattern was believed to be a superimposition of the early depolarization and repolarization consequences of the PMI. The significance of this observation lies in the ECG detection of PMI, frequently underdiagnosed particularly in patients with an atypical presentation, and with the RBBB adding further to the complexity. Thus, it is gratifying to note the contribution of the ECG to diagnostics, the only modality that provided a pathophysiologic insight in these two patients who appeared to be abruptly deteriorating clinically without an apparent reason.