Abstractlnterlayer dissection of the atrial septum resulting in the formation of a cavity developed after an attempted repair of a mitral paraprosthetic leak. Subsequent rupture of the cavity into both atria resulted in a small left‐to‐right shunt and significant left ventricu‐loatrial regurgitation. Misplaced stitches, aimed at obliterating the paraprosthetic leak, were the cause of the dissection. This case illustrates a previously unreported complication of mitral valve surgery and stresses the importance of proper exposure and handling of the mitral annulus. (J Card Surg 7994;9: