"Upgrade denied" can also be found as follows:

1. You buy a C class international fare for a trip such as ORD-SFO-NRT, and are booked in C on the international leg SFO-NRT and A (first) on the connecting domestic ORD-SFO leg.

2. You apply an SWU to the entire trip, and succeed in getting NF on the international leg. The domestic leg comes back "upgrade denied," since you cannot upgrade an A fare -- it's already in confirmed F.