As far as I know, most if not all airlines require their Flight Attendants to sit in the brace position for take off/landing and do a silent review. However, came across this video from an Olympic flight (the part im talking about is from 42 seconds onwards)....
I understand the guys just having a bit of fun, but would this actually be considered a big deal by a CSM/Purser/Lead Flight Attendant? would the consequences be just a short talking to, or more serious?