Two recent cases:
PRN-VIE, almost full flight in Y and one pax in C (most probably, a staff member of the EC, as she carried her Laissez-Passer like the most precious item that has ever been discovered). I was in Y, sat in my 3C seat, with row 2 being used as C class but empty. When the boarding finished, the purser came to me with the usual "thank you for flying" and suggested that I move to row 2 but highlighting that she had only one meal for C. I thanked for the offer and said that even if was in C, I would not be eating, but appreciate extra space. So, I moved and sat at 2A, while the other pax was in 1F. When the 1F came back from the restroom, she noticed me, called the purser and said quite loudly "The passenger in row 2 does not belong here. I checked the at the check in counter, I am the only BUSINESS class passenger". I don't know what the purser said but she turned back to look at me and turned away, not being too happy and said again "But this is business class. People from economy cannot sit here." I went to the purser and said that I am OK to move back to my seat if this is causing some issues. The purser said "Oh please don't. It's all fine." During the flight I was offered full bar and some spontaneous creation from the crew: bread, fruit and nuts. We landed, of course remote stand and... I saw her again on my flight to BRU, both of us seating in Y.
ZRH-GRU, as always pre and post C19, full in all classes. I was in F and was picked up from the remote stand and driven to the gate, When we got to E terminal, I thanked the driver and suggested that I can make my own way to the gate and she can go back, as it was already late. When I got to the gate, the boarding was mid way, with F must have already boarded, as F/HON line was empty and C was moving ahead. I went to F line and stood there for 5 seconds. The pax from C line that I was supposed to use to scan the BP, says "This is only for business class. Economy is not boarding yet. Are you in business class?" I replied to the passenger "No, I am not in business class." The agent saw me, waved at me, scanned my BP and said "Enjoy our first class" and winked at me.