On a recent short KLM flight, we were told that we should stay in our designated seats until the seatbelt signs were off, and that if we wanted to move after that, we needed to check with cc first, for trim reasons.
Two or three people completely ignored these instructions (worrying when the express reason given relates to safety - that level of ignorance/arrogance disturbs me) and were sent back to their original seats VERY loudly by the cc.