I run slime tubes and the last two days I have had overnight flats after installing a previously patched tube. Both tubes had a small hole on the inside of the tube about one half inch from the valve. I have checked the wheel liner, it appears to be ok and there are no foreign objects on the liner. The hole is too far away from the first spoke on either side of the valve opening. When I install the new tube I screw the nut on the presta valve as tight as possible but, I have noticed that while inflating the nut on the presta valve was not as tight as thought (the inflation creates about a 1/4 inch gap between the wheel and the nut).
The only thing that I can figure out is that while I am inflating the tire I am somehow pinching the tube at the valve.