'Defective tubes' are often blamed for flat tires, but in my experience, truly defective tubes are extremely rare. Especially when it is a flat caused by damage around the valve stem, the most likely cause is rough handling during pumping, or when detaching the pump from the valve. Another cause can be running pressure too low, which allows the tire and tube to shift, but the valve stem hole remains in the same place and this puts stress on the tube. There are tubes that are better and worse in this regard (often more expensive tubes are better, IME), but actual 'defective' tubes are rare.