I was on UA with my brother. I had upgraded to C but he hadn't gotten his, on a 3 cabin transcon. They insisted there was no space.

The plane took off and there was a seat available in C, several in fact. I politely asked the purser and she first turned me down. Then I pleasently insisted and she went and got my brother and brought him to C and let him sit there. They never did collect coupons from him when we left