This happened to me a few weeks ago. I went to check my tires before my commute to work and my back (why is it always the back?!) tire was completely flat. So I change the tube and, sure enough, the sidewall of my tire is buldging out. I rode gingerly to and from work without taking any extra loops on the way home and got some new tires that night.

I think this is something that you just have to expect once in a while and be prepared with a spare tire at home.