My first thought, like many of you, was that everyone knows (or at least all frequent fliers know) that missing a leg will usually result in cancellation of you

My first thought, like many of you, was that everyone knows (or at least all frequent fliers know) that missing a leg will usually result in cancellation of your flight, and that you have to let the airline know BEFORE the first flight departs that you don't plan on taking it. The OP could have saved himself a lot of trouble by simply calling United before he left and canceling his departure. After mulling it over, my second thought was that air travel is a BIG DEAL to most people -- it's stressful and expensive, and the rules are opaque. I agree that United could have made this MUCH better by sending the OP a notice within, say, 12 hours of missing his first flight explaining the situation and his future options. Straying from the customer experience, on a systems side, airline yield management is a TERRIFIC problem to think about. By forbidding passengers from skipping flights in an itinerary, you effectively remove the restriction that the total itinerary price must be the sum of the legs that it comprises. This opens up all kinds of new possibilities to use economics to balance load across the system.