BACKGROUND: Diagnostic errors or delays can cause serious consequences for patient safety, especially in the emergency department. Anchoring bias is one of the major factors leading to diagnostic error. During the coronavirus disease 2019 (COVID-19) pandemic, the high probability of COVID-19 in febrile patients could be a major cause of anchoring bias leading to diagnostic error. In addition, certain evaluations such as auscultation are difficult to perform on a casual basis due to the increased risk of contact infection, which lead to inadequate assessment of the patients with valvular disease. Acute mitral regurgitation (MR) could be a fatal disease in the emergency department, especially if there is a diagnostic error or delay in diagnosis. It is often reported that diagnosis can be difficult even though there is no treatment other than emergent surgery. The diagnosis of acute MR has become more difficult because coronavirus disease 2019 (COVID-19) pandemic could affect our daily practice especially in febrile patients. We report a case of a diagnostic delay of a febrile patient because of anchoring bias during the COVID-19 pandemic. CASE PRESENTATION: A 45-year-old man presented to the emergency department complaining of acute dyspnea and fever. Based on vital signs and computed tomography of the chest, acute pneumonia due to COVID-19 was suspected. Auscultation was avoided because of facility rule based on concern of contact infection. After admission to the intensive care unit, Doppler echocardiography revealed acute mitral regurgitation, and transesophageal echocardiography revealed mitral valve tendon rupture. After confirming the negative result for the polymerase chain reaction of severe acute respiratory syndrome coronavirus 2, mitral valvuloplasty was performed on the third day after admission. The patient was discharged 14 days after admission without complications. CONCLUSIONS: In COVID-19 pandemic, anchoring bias suspecting COVID-19 among febrile patients becomes a strong heuristic factor. A thorough history and physical examination is still important in febrile patients presenting with dyspnea to ensure the correct diagnosis of acute mitral regurgitation.