Hilly Hundred on my touring bike, an Assenmacher (vintage 1980), while my youngest son, Justin, was riding a much newer Giant Defy 3. (We had a a great time, BTW).
In any case, at about 44 miles into a 50 mile day (which would be a 57 mile day). I had my first flat. While descending at 30+ mph on a quarter mile long hill. Thankfully, it was merely exciting, and nothing more. Having ridden said bike for more thank 40,000 miles over the last 30 years, probably helped, along with the fact the hill was straight, and I could get stopped and to the side of the road without a lot of interference from other bikes.
I get off to the side of the road, pull my tools and spare out of the rack trunk, lay the bike down, and pull the front wheel. First, I begin looking for the thorn or cut that was sure to have caused the problem. Nothing there. So off comes the tire, I pull the pump, pressure the tube, and find the hole. It lines up with the sidewall, and a small cut there. An not just a cut, but groove about an 1/8" above the rim, All the way around the tire, down to the cord. And a "D'oh" light bulb came on.
Throughout the day, I had been hearing a soft, intermittent buzzing sound. I knew it was on the front of the bike, but I couldn't find the source. It wasn't spare camera batteries, loose change or other camera gear in the handlebar bag. It wasn't fender or rack hardware. (Like I said, it is a touring bike.) But I just didn't put it together that the gentile buzzing was the top edge of the front brake pad gently touching the tire sidewall just above the rim. And each time I applied the brake, I cut away a little more sidewall and tire cord. So finally at 44 miles, enough cord was gone to let the tube through and cause a blow-out.
It all went back to a few days before Hilly, when I had the bike in a car with the front wheel off. In hurry, I had just shoved the wheel back into place, without releasing the cantilever cable. And my vintage Deore XT cantilever, 3-plane adjustable pad had twisted up and away from the rim to contact the sidewall.
Rushing, rookie or just dumb, I was fortunate I just had two flats and no crashes. Did I mention the second flat? Seems like the casing gave away a little more about 10 miles later, and I finally had to boot the tire with a folded $5 bill (no $1s in my wallet) to finish the last 5 miles.