Meanwhile, the Air Force, which has a $156 billion annual budget, still doesn't always use serial numbers. It has no idea how much of almost anything it has at any given time. Nuclear weapons are the exception, and it started electronically tagging those only after two extraordinary mistakes, in 2006 and 2007. In the first, the Air Force accidentally loaded six nuclear weapons in a B-52 and flew them across the country, unbeknownst to the crew. In the other, the services sent nuclear nose cones by mistake to Taiwan, which had asked for helicopter batteries.