I've had my carry-on bags weighed in Europe, North America, and Asia -- like others have said, it depends on a variety of factors as to whether or not they'll single you out or not. Size and weight are two different factors -- bags can be checked at the gate if they are too big, but if they are too heavy have to go in the hold. I know with a couple of airlines it's a ploy to get more money out of you. If your bag is too heavy, it has to be checked. That puts it under checked bag allowance -- which likely means it will put you over the weight or piece allowance, in which case you have to pay the excess baggage fee. That being said, out of hundreds of flights flown, this has only happened a handful of times, so more than likely you'll be okay.