There are a couple of anomaly locations where you can fly for intra-US mileage. Sounds exciting at first, but it is not really a great deal. Even if everything books right and is ticketed properly (a big if), then you can travel to an obscure location in Asia for 50,000 miles in business round-trip. What's the fun? And if you go one way and drop a segment, then you risk a follow-up, and you need to pay for your way back anyway. No fun either.