Two, or kinda the same one twice. While riding RAGBRAI I got a small cut that looked like it came from glass though I could neither see nor feel anything in the cut. So I replaced the tube with a new spare and that was that for a couple of weeks. Then the new tube got a flat in exactly the area where it touched that old cut. The new hole in the tube was pretty much the same size and shape as the cut in the tire. Again I could see and feel nothing there and I used a hand held microscope this time. So I patched the tube and then put a stick on patch on the inside of the tire over that cut for good measure. I don't know if the "sharp" lip on the cut on the tire abraded the tube, grit working through the cut abraded the tube, or if there is a wire in there that defies any attempt to see/feel it.
Ken