We here present a case of a perimitral flutter occurring after a prior atrial fibrillation cryoballoon ablation and a second ablation procedure using radiofrequency with Marshall-PLAN study protocol (ethanol infusion in the vein of Marshall plus endocardial mitral isthmus, roof, and cavotricuspid isthmus lines). Activation mapping revealed mitral isthmus and roof reconnection. However, endocardial ablation at these sites failed to terminate the arrhythmia. Complete mitral isthmus block occurred only after radiofrequency applications at the great cardiac vein level. Epicardial connections at mitral isthmus sustained the arrhythmia and prevented block. This case underlies the importance of evaluation of mitral isthmus block with proper validation of ablation lines to prevent future tachyarrhythmias recurrences. [ABSTRACT FROM AUTHOR]