Here's mine.

Chongo and I did a 15-mile up and down hike near where we live. Knowing the terrain well, we brought along just enough water (2 qts. each). We got to the peak (7.5 miles) with no problems, and with just a quarter of our water left. The sun was blazing (about 100F), there was hardly any shade anywhere, but hey, no problem, it was just going to be downhill back to trailhead. Then my knees gave out -- and every single step was excruciating pain!

What took 3 hours to hike up took me 5-6 hours to hike down! Not anticipating this at all, and with the sun pounding down on us, we both finished our water pretty quickly.

LUCKILY, I saw another hiker on his way up, and asked for a bit of his water. The good samaritan shared his water with us, then told us where he had hidden an extra quart a few miles downhill. He then turned back to go downhill -- passing us quickly behind. We eventually reached his stash of water and quickly consumed it all. Now, just a few more miles to trailhead!

AMAZINGLY, the good samaritan actually got down to the trailhead before us, filled up yet more water, and then hiked back UP the fairly steep trail to bring it to us -- all in 100F heat!!!

Lesson learned -- always bring a bit more water than you think you need!