Pulse oximetry detects hypoxemia non-invasively by measuring light absorption at different wave lengths, depending on the oxygenation of hemoglobin. The plethysmographic signal is caused by volume changes in the arterial blood during systole. In this pulsatile flow, oxygen saturation is calculated using an algorithm. Use of the pulse oximeter has increased massively, especially in the COVID-era, and has a central role in managing patients at the emergency department. However, interpretation of the plethysmogram is frequently incorrect which may lead to incorrect conclusions. Moreover, an irregular plethysmographic signal may well 'give' a saturation percentage. We will discuss how pulse oximeters work and discuss some pitfalls. We hope that understanding the mechanism behind pulse oximetry, as well as an awareness of pitfalls, leads to improved management of patients with consequently less unnecessary care, as demonstrated in our case.