Coronary artery fistula is a rare condition where a fistulous communication exists between the coronary artery and a cardiac chamber or vein. Mostly they are of congenital origin, but rarely may be acquired due to trauma, post cardiac surgery or angioplasty, or very rarely, because of coronary aneurysm rupture. Ours is an interesting case of a post-coronary artery bypass graft surgery status with a fistula in second obtuse marginal branch of left circumflex coronary artery draining into left ventricle aneurysm. We explore possible causes including erosion of the thin aneurysmal wall into coronary lumen. Given the history of previous surgical revascularization, it may be a contained rupture and the possibility of pulsatile effect of the obtuse marginal branch eroding into the thin wall aneurysm is considered.