I've only grayed out once, and that was on Millennium Force. I was sitting in the very last seat and started to gray out about halfway through the ride. I don't know if it was because I had worked an eight hour shift before going to Cedar Point, or if it was because it was the first ride I went on, but it freaked me out a bit because it had never happened to me before. The funny thing is, it hasn't happened to me since, not even after I rode it 10 times in a row last year at Boo Buzz.(I'll never ride it 10 times in a row again, it made me feel a bit dizzy when I was done.)