You have put a 7kg (most of the time it is more) carry-on bag in a locker above your head. Chances are, it will move.
Watch out.
Hah! Heathrow, T5 last week, a passenger was struggling to lift a bag up to get it in the overhead. We couldn't get it lifted either, and insisted we gate checked it into the hold! (I'm ground staff, I was just on board to check a seat!) We labelled his bag and got the loader to load it for us! There was a yelp from the bottom of the stairs when the loader put the bag on the scales - 37kg! Is that a world record for an attempt to take an unsuitable cabin bag?