I've been on international flights where this happened; a combination of the cargo and fuel loads, interacting with the passenger load. Put the passengers in the wrong places and the plane won't sit right in the sky. Generally this will only affect fuel economy, but it can be a 'contributing factor' to disaster, reducing safety margins. Read
Doomed for a detailed report on what really went wrong when the Concorde crashed, describing how weight distribution plays into a crisis.
On one of my Alitalia flights where this was an issue, people just reseated themselves in the air, in a trickle that turned to a flood, with no crew intervention.
(No surprises here; Italy is my favorite country on earth, but I swear that Napoli-bound passengers stand to get their bags as soon as the front tires touch the tarmac.) Undoubtedly the seating precautions in place were most critical for takeoff.
Only a small fraction of the population is capable of designing decent safety protocols. I don't have any particular faith that these are the people whose rules we actually experience, but when someone breaks safety rules because they can't imagine the harm, I'd prefer that they were only offering themselves for a possible
Darwin Award. As a mathematician, I'm constantly reminded that lack of imagination never proves anything. On a plane, we're all in it together.
Take the seat you're assigned, until the fuel weight drops and you're told it's ok to reseat yourself.