The following atypical features of activation and entrainment mapping may be encountered during the diagnostic mapping of atrial tachycardias. (1) Unusual disparate activations of the coronary sinus and the contiguous left atrium, which can be deciphered by magnifying the recordings on the coronary sinus bipoles. (2) After entrainment of tachycardia, the return cycle length (postpacing interval) may seem shorter than the tachycardia cycle length. (3) Despite the site lying close to the reentrant circuit, the return cycle length (postpacing interval) after entrainment may be much longer than 30 ms because of altered conduction in the local area.