Originally Posted by
jeanlucsf
I am not sure if you answered my question #2. Is it also a UA display error if flight KOJ-HND is available in Eco, flight HND-SFO is available in Business, but the combined KOJ-HND-SFO is not available?
The search engine doesn't always find all applicable options. You may be able to book this using multi-city search. You may be charged extra miles, unfortunately.
In my experience, UA's search engine seems more likely to return flights where the economy segment is after the business segment than the reverse. No idea why that would be, but I've noticed a pattern.