I was in the gym going about my business, and I got to the point in my routine that called for squats. Not squats with ridiculous amounts of weight, just enough to work a little. I did three sets, as I got done with the last set, I placed the bar back on the rack...and both hamstrings simultaneously cramped up so hard I thought I was going to fall over. I hobbled around on my seized up legs for a while, until I was able to stretch them out enough to become functional again.
I don't know if I was dehydrated, or it was from not squatting for a while, all I know is that I am one sore pup today. That is until I went on my run, after I got warm, my legs felt great, real strong and smooth. The body is a strange thing.