I had that happen last weekend when I was adding some air to the tires after doing some other maintenance. Kinda surprised me, because I didn't realize that the tubes I've been picking up for $4 new at the bike co-op had removable valve cores. Now I know, I've tightened up the cores and have been a little extra careful to make sure that only the very tip on the valve gets unscrewed the last couple times I've topped off the air in the tires.