Well, once officially.
A second time on a ride without official SAG support.
First time was the 2001 MS150 from Boston to P'town. Day 1 was an optional century, which DH (then, DBF) and I attempted. It was blisteringly hot out (95, but much hotter on the pavement), and the MS crew had a snafu with the water stops on the century spur, so there was one less water stop than they planned. The heat and running out of water got the best of me, and at the 80+ mile rest stop, we rested in the shade, but couldn't get my HR down below about 120. Heat exhaustion. I was wiped. Couldn't go on. Really on the verge of tears, if I had any liquid in my body to cry out. So, I took the van back to the Mass Maritime Academy (the overnight stop) and DBF/DH sprinted the final 20 miles back, so he completed his first century. It would take us 2 other attempts before we were able to finally complete our first century together.
Oh...and I took a long cool shower, had a few glasses of water, then got to work on the beer with our team, and rode 75 miles the next day!
Second ride was during the Sunday ride with our LBS. I was on a borrowed shop bike (long story) and the group of us were screaming down Beach Drive through Rock Creek Park. One of the idiots in front of me grabbed a handful of brakes and zigged around a pothole, rather than bunny-hopping it or calling out, and clipped wheels with the guy behind him who went down, but not before taking me out, too. So the two of us crashed - him onto the road, and me into the gravel on the side of the road. I was majorly hurting, and the wife of the LBS owner (who was not riding at the time...I think she was pregnant), came and picked me up with her van and took me back to the shop. I was so upset, because I scratched the brakes on the borrowed bike and ripped my favorite pair of shorts!!