I read one incident a few years back when a young and healthy CPL student went up with a fellow student to do some CPL revision after a heavy night out on the town and G-LOC'd doing a max rate turn in a 172. Fortunately the other student was flying at the time so they made it back in one piece. The incident report stated that fatigue, the lingering effects of alcohol consumption, dehydration and a lack of recent experience with sustained G maneuvers were contributing factors.