Misplacement of electrodes can change the morphology of an electrocardiogram (ECG) in clinical important ways. To assess the frequency of these errors in different clinical settings, we collected ECGs routinely performed at the cardiology outpatient clinic and the intensive care unit. Lead misplacement was suspected when one of the following morphological changes occurred: QRS axis between 180 degrees and -90 degrees , positive P wave in lead aVR, negative P waves in lead I and/or II, very low (<0.1 mV) amplitude in an isolated peripheral lead, or abnormal R progression in the precordial leads. We analyzed 838 ECGs and identified 37 ECGs suspicious for electrode misplacement, from which 7 were confirmed. The frequency of ECG artifacts due to switched electrodes was 0.4% (3/739) at the outpatient clinic and 4.0% (4/99) at the intensive care unit (P = .005). In conclusion, errors in ECG performance do occur with an increasing frequency in an acute medical care setting.