The wrong clothes
Today, without much warning, it was suddenly snowing heavily, all through my run. As Billy Connolly says, "There's no such thing as bad weather - only the wrong clothes."
Today I had the wrong clothes. I got soaking wet with snow and freezing rain. I had to wring my gloves out and windmill my arms around to keep my hands warm - and keep running to get home.
When I got back, I realised I was a little hypothermic - all the blood was retreating from my extremities to keep the core warm. After my shower, my skin was bright red with the blood coming back to it.
Lessons to learn from this?
Take a shower-proof jacket in my pack, and buy goretex gloves.