Just happened to me last weekend. Was on a partner award from BOS-AGP, with the only segment in 3 cabin 1st from EWR-BRU. Should have realized something was up when I show up at checkin in BOS, and they can't print out boarding passes for anything but the first leg. Tell me to talk to the gate agent in EWR.

When I get to the gate at EWR, the two GA's are muttering about how this guy can't be in first as he only paid $94, it must be an economy fare, lets just put him there. I realize they are talking about me, and let them know I know........point out I have a first class seat on a partner award. "sorry we don't even have an O class so we're putting you in economy"

Eventually get to talk to the concierge guy, who tells me i'm not flying in 1st regardless of what I say, he can get me in business and give me a voucher, but it's the only way i'm getting to AGP in the next 2 days. (Too close to risk taking bags off at this point, I had checked my golf clubs)
I asked about checking for volunteers, and he pretty much laughed at me, and said, look you are the lowest value customer in the cabin and we'll be moving you down, take it or leave it.

My status is on other airlines, AA and DL after I vowed never to fly UA again, I was using up the last of my US miles to get rid of them as I got so fed up, and now I have to fly UA at least once more to use my voucher, the irony is killing me!