I just love when I make a PA reminding them to keep their seatbelts fastened (after I have been released from my seat after take-off) until the captain has switched off the seatbelt sign. Pretty much as soon as i hang up the handest i always get at least one pax jump out of his seat for something totally important out of the overheads...
Also when passengers are getting on board and they are trying to jam ridiculously sized items of so called hand luggage into the overhead lockers. Bags etc that clearly are not going to fit and they push and push and they look at me (still boarding passengers and checking boarding passes), but they look at me and sometimes i really have to keep myself from saying 'what do u expect me to do, i cant make the overhead locker any bigger!'