I went on a group ride and let my ego make the decision to ride with the fast guys. I can usually handle 18 - 20 in a pace line. These folks were riding 21 - 22. I've been doing wind sprints 3X week and thought I could hang. At ~20 miles I got dropped and they waited for me. At 25ish miles it happened again. I didn't want to mess up their ride so I got directions back to our post ride coffee shop, thanked them, and began to limp back. One of the guys rode with me about a mile until my turn off and I told him I'd see him at the coffee shop.
Next thing I remember, I woke up 20' from the road laying in some shady grass. I could tell by how my bike was placed, and that I was in the only shade around, that I had chosen this spot as opposed to crashing. I downed a Gu, some water with electrolytes, and made my way slowly back to our post ride coffee shop. Looking at the miles and time, I've got about a 35 minute hole in my memory.
Three days later I did the same ride with the same guys at the same speeds. Both rides were in the early morning of days where the heat index was north of 100. This time I drove to the start point instead of riding (7 miles). I also ate about twice as much, and drank an extra bottle of hydration en-route. This time I finished strong and had a great time.
I'm not sure what happened the first time. When I decided to throw in the towel I was completely convinced, and convinced the other riders, that I was more than okay to slowly ride back. I still had hydration and nutrition with me. I guess from now on if I need to drop I should call my wife and get her to come pick me up.