It's about people who legitimately get a chair on the deck by the pool, actually use the chairs, but have one of the chairs taken over by people who get up late, see a chair that has been vacated for a few minutes and try to just sit down and use it.
I have had people try to take my DW's chair when she got up for a few mintutes to go to the bathroom (but I was still in my chair beside her). It was like fighting off a pack of wolves. I really don't think people really wait 30 minutes. They get up at noon, and use their arrogance to snag an open chair near the pool even if it's only been vacated for only a few minutes. It's happened more times than I care to mention.
Let me clarify that we never sit in the Serenity area.
Happened to you?
Carnival 11 times (12th booked for June 2018)
Princess 3 times was awesome
Big Red Boat 1 time too many