Front tire was flat after leaving my bike at work overnight. I thought someone let the air out as a joke. I looked over the tire and couldn't find any damage. I pumped it up to about 60 psi which is all my cheap little pump will do. I got about half way home of my 18 mile commute and noticed the tire was going soft. Stopped and pumped it up again. Made it home with minimal fuss. I pulled the old tube and quickly found the leak. Inspected the tire again and did not find anything that could have cause the flat. I did notice that the tube was too small for my 700 x 35 tires. The bike shop didn't change tubes when installing my new tires a couple of months ago. The tube was only rated up to 700 x 32. Maybe that caused the problem? New tube installed and no problems now.