I've had two tubes fail at the valve. After the second failure, I took a a good look at the valve hole and determined the bored edge of the hole had a sharp edge that was cutting into the tube. I ended up chamfering and softening the edge by hand with a piece of sandpaper. I also re-enforce the tube at that location by cutting a postage size piece from an old tube, cutting a tiny hole in the patch and pressing it over the valve and gluing it in place.