I always carry two spares on my bikes with tubulars. Last week I was a few miles from home on my return commute when I came upon another cyclist stopped so I stopped and asked if he was okay. He'd gotten a flat and was hoping to flag down a car to take him the few miles to Belmont Wheelworks. (It wasn't going to happen on that road!) All I had was two spare tubulars but after a few moments reflection I recall that someone in C&V had described putting a tubular tire on a clincher rim. So we tried it. He made it safely and I then retrieved my tire and went on home. Now I've started carrying a patch kits even on the tubular bikes.