Originally Posted by mherdeg

As long as the operating carrier shows all their segments in their view of the reservation, you should be ok.

It's common in multi-carrier bookings to be "missing" other airline segments; if you buy a ticket from UA with travel on TG/LH, you'll often see that the TG view of the reservation shows all the TG flights plus the one other-airline flight immediately before the UA flight, but is missing the others. This is fine.

If any carrier's view of the trip is missing flights on their own equipment, you have a problem. (If UA thinks you have a NH flight but NH disagrees, this is bad.)