One of my worst cramps occurred on a 100 km ride in January 2010.
I had developed DVT and lost all my fitness a few months earlier, and was building back up again.
That day was over 30C by 8 am ... and hit something in the 40s by the middle of the day.
The ride started with a long, long climb ... which I handled (but during which time I didn't drink much), and then it flattened a bit so that was OK. Then, during the hottest part of the day, we tackled the steepest climb of the route, about 3 km at about 12% (and again, I didn't drink much). By the time I reached the top, I was not feeling well at all. I had been under the impression that the route flattened after that, but it was rolling hills. I reached the next town, but I was dizzy, nauseated, and well into heat exhaustion.
Rowan and I rested a bit, drank a bit, and had a bit to eat ... and then we kept going, stopping every couple km so I could lie down on the side of the road and feel like I was going to die. And then my quads started to cramp mildly.
At one point Rowan cycled off ahead a little way, on the way up another hill, and of course my quads chose that moment to seize up entirely. I had managed to get off the bicycle just in the nick of time, and the only thing I could do was stand there. I couldn't walk. I couldn't get back on the bicycle. All I could do was stand.
So I'm standing there on the side of the road with my bicycle next to me, sipping water in the hopes that it will help, when all of a sudden a car pulls up and stops in front of me, and a young man hops out. "Oh lovely", I think, "Either this person is here to help ... or, if it is someone who is going to hassle me, I can't run away!!" But all he wanted was directions. I stood there, completely unable to move, and gave him directions to the next town. And then he drove off.
It was 5 or 10 minutes before I could start hobbling down the road.
Shortly after that, Rowan cycled off to get the van, and I walked several km to the next town. I could get on the bicycle and coast down hills, but there was no way I could ride up hills. I ended that day with 88 km.