The only time this ever happened to me was when I had lost the little nut that goes around the Presta valve against the rim. So, when I was pumping the tire, the valve got pressed into the tube and split a leak right at the tube, which of course I didn't realize until the next day when I got my bike out of the car to ride, and am embarrassed to say that I didn't have a spare tire on me. (This is an easy fire road/doubletrack trail and I've never once gotten a flat on my mtb so had gotten a bit lazy). So, no ride for me that day. Lesson learned!