I was unupgraded at the gate once on Delta, haha. I was flying out of LAX so I wasn't expecting an upgrade at all, and I had exit row so I was fine anyway, but when I checked in I was given a boarding pass with a first class seat on it. So I double checked it and was apparently upgraded.
So I go to board when First Class is called, and when they scan my boarding pass a horrible beeping noise erupts from scanner machine. The gate agent then very rudely starts asking me about the boarding pass, where I got it, etc. So I explain to him, and he continues with the rudeness and confrontational attitude. He even goes so far as to say something like "I have Diamonds on this flight who weren't upgraded, why would an Alaska elite be upgraded". You should have heard the disgust he put into his voice when he said Alaska elite.
Best part was that I lost my good exit row seat, with nothing in front of me, and got stuck into that weird exit row window seat that has the wall jutting out in front. Being that I'm 6'9, it was fairly uncomfortable. Thankfully the person next to me didn't mind my legs being cocked across at an angle.
Anyway, as soon as I sat down on the plane I started tweeting with Delta's customer service, and ended up getting a $200 voucher, which I have to give them credit for, but it is the first time I've ever been unupgraded before, and I'm still not sure what would happen that would cause their system to give me a seat like that.
To clarify: I wasn't even really mad about losing the upgrade, like I said, flying out of LAX as a partner elite I wasn't expecting to get upgraded anyway, it was just the incredibly rude way that I was treated that got my hackles up.