Lol this happened to me flying business class from Brisbane to Sydney just before the pandemic. I normally fly economy but had to get a last minute urgent ticket to get back to work the next day.

Boarding already started when I arrived at my gate. Joined the priority line instead other regular line which extended around a bend. The priority line often allows people with babies or the elderly to also use it to check in. Two women in their 40s were like "oh looks like anybody can join any line it seems" and they stood in line behind me. They were passive-aggressively talking about me leading up to the final ticket check place. All kinds of subtle digs and trash-talk.

As I was let in, I could hear the staff advising them that normally its reserved but they must have let them through because they were soon behind me leading up to the main plane too. But I remember the look of dismay and shock on their faces as they had to wait while I took my seat literally in 1A, and a hostess put up my backpack for me.

"Welcome to business class Mr DnANZ. Would you like a drink?"

I don't know if they they asked if people were getting random upgrades, but I like to imagine they did. I put on my headphones at that point.