I have had horrible cramps to the point I could not unclip and had to lean against a fence. I've cramped driving home from a ride when I couldn't lift my leg to push the clutch pedal. I've cramped so bad lying on the floor at home that I couldn't get up.
I never cramp, however, when I have been riding enough, no matter what hell-ride I do, as long as I am hydrated. I never stretch, have a terrible diet, and am at least 10 pounds overweight but I will try almost any climbing ride up to around 125 miles. There may be a genetic component involved.