I learned a small lesson yesterday: If your tires are worn, carry a spare!
I've had great luck with the 28 x 700 Continental Gatorskin tires. The first 2000 miles resulted in two flats. Considering the combination of ride, speed and durability, I consider that to be a good result.
Sunday I got my third flat. Thursday I felt a slight bump, bump, bump, with each turn of the rear wheel. On inspection I found a bulge on the tire. So I reduced the air pressure and headed directly to the nearest lbs, a Performance store about 5 miles away.
While the selection was good, They did not have a folding Gatorskin in any size. So I purchased a 700 x 25 Ultra Race from Continental. This is a light & fast tire, but not as puncture resistant or as durable as the GP4000S or Gatorskin. Not cheap either, I hate spending money on something I don't really want to buy.
So I installed the Ultra Race and finished my 70 mile ride.
I'll buy another Gatorskin and remove the Ultra Race and keep it as a spare.
Once a tire is more than 75% worn, something is going to go wrong. The possibility of flats increases due to thinner tire and damage to the structure of the tire becomes an increasing risk.
The next time a worn tire comes to the end of it's life I won't need to make an emergency stop at the nearest bike shop, I'll have a spare.