A 19-year-old girl was referred to our cardiology department for catheter ablation (isolation of the pulmonary veins) of paroxysmal atrial fibrillation (AF). The diagnosis was made upon a 12-lead ECG of the arrhythmia documented in the emergency room. The ECG showed an irregular tachycardia without wide QRS complexes. Careful assessment revealed the irregularity of the rhythm was a sweep artifact due to a mechanic failure of the ECG-machine to advance the article smoothly. During EP study a concealed anteroseptal accessory pathway causing an orthodromic AV reentrant tachycardia was eliminated by radio-frequency ablation. This example emphasizes the need for careful assessment of an ECG tracing, including printed legends and technical data.