I once locked my bike outside somewhere, got drunk, returned to bike at 2 AM so as to ride to some party across town. found that I had two flat tires, walked bike 9 blocks home, and attempted to patch the flats. Patched both tubes, tried to pump one of them up, BANG. "What? tire looked like it was seated." patch tube a second time, pump it up, BANG. hmm. move on to other tire, try to pump it up, BANG. I am bewildered until I try to repatch and pump the tube yet again, and actually see the knife hole in the tire expanding as I pump, and can see the tube sticking out of it. although I can't pull the pump off the valve in time to let the air out of the tube. BANG. needless to say my roommates were not stoked. lesson learned: don't lock up your bike on the path between the frats and the campus bars at night, in fact do not lock your bike up at night period.