Premature ventricular complexes (PVC) often are useful in ECG interpretation. Sometimes, in patient with long PR interval (first degree AV block) the P waves seems to merge/fuse with the T waves. If you look for a PVC, there you will see the distinct P wave and you are now convinced that there is sinus rhythm with long PRI (first degree AV block)
Image 1 - A 12-lead ECG misread by machine
