I did once in high school. I stopped for gas thinking I had a $20 in my wallet, and it turns out that I didn't have any money at all. I left the lady my wallet except for my DL, ran home and grabbed a $20, and went back and paid. I felt like such a doofus. Now that I have a CC, I pay at the pump.
I also found out the hard way that most fast food places don't take CC's when I was in college. I stopped to get dinner at Long John's, and I had NO cash at all. They ended up just giving me the food and telling me to have a good night. Again, I felt like a huge tool.
Ryan