Anecdotally I would say I see this kind of thing happen on about 25% of domestic flights here in the US, cant help feeling that its driven by many people flying Southwest and taking that open seating mindset into other carriers. I've even seen a person plonk themselves in 1A on the 1-2 config and then have to be shunted behind the curtain back to economy, which takes a certain amount of Chutzpah or ignorance.

While each situation merits its own reaction, mine typically is that I chose this seat for a reason and I'd like to sit in it thank you.