Flying a lot UA TPAC flights between LAX & PVG, I've seen several passengers from the China side try to sneak into Business First when the FA's weren't looking. One time on a 1/2 empty cabin in BF, I fell asleep a couple of hours into the flight, and woke up to a Chinese gentleman sitting in the adjacent business seat.

I got a little suspicious, so got up I asked the FA if somebody in Business decided to move their seat assignment next to me. She said no, so I asked who was the guy in the seat next to me. She then asked for the guy's boarding pass (in English) but he claimed (or feigned) ignorance of the language, so the Chinese speaking FA came over and asked the same question. He pulled it out and it was for an economy seat. When she told him to move back, they had an argument in Chinese that lasted for a couple of minutes. He finally went back to his seat escorted by the FA.

When she came back, I asked her what they were arguing about. She said he refused to get up because the seat was too comfortable and he already had settled in there, so there was no reason for him to give it up, especially if no one was using it. She said that she explained he would have to pay for the seat, but he argued that no one was using it anyway, so why does he have to pay? So she said he had three choices; 1) pay for the seat, 2) move back to his original seat voluntarily, or 3) Be forced back to his original seat, and be arrested upon landing.