Answer: when plane is diverted due to medical emergency. HKG-sfo small child having seizures a few hours out - this was not announced I know b/c I'm an MD. Why they deserve congrads: This must have cost SQ a fortune - pilot came on to announce diversion to NRT and that we'd have to circle a while to dump fuel. (can't begin to think what that cost) Landing fees at nrt I assume aren't cheap. Interesting - there was a fire engine and ambulance about every half mile as we taxied to the terminal The A380 engines need time to cool down - and we were informed about this and kept up to date about events in a timely and not intrusive fashion (eg diversion wasn't announced by the pilot until shortly before we were to land b/c most passengers were sleeping). Crew announcement at one point "thank you......... WE'RE ALL IN THIS TOGETHER", crew helped each other out, worked during their break time - a real team effort. By luck arrived at sfo 30 minutes before immigration closed (1am) - does anyone know what they would have done if we landed later? Stuck on plane for 7 hours or make immigration do overtime?