I once returned from Mountain Biking in West Virginia, tire still inflated when I rode back to the house where I was staying. The next morning my tire was flat. I actually had 3 punctures picked up on the trail that were so tiny that all three of them together only amounted to a slow leak. Culprit: Tiny hair-like thorns that are so soft and willowy you couldn't manage to prick your finger with them if you tried. But when they get in your tire and work their way through such that just the tips protrude through the casing, the tire itself gives them enough support to puncture.