Depends on the airline and how full the flight is, but *damn* the flight attendants/crew can crack down. Especially on the later-boarding passengers. Not that it's the late row passengers fault. The overhead bins are full with all the early boarders stuff!
When I've been in the unlucky, late boarding rows, even with a carry on bag that below the regulation size, I've twice had a flight attendant - right at the door of the plane - go, SORRY! we're gonna have to gate check. Bins are full. And they just grab that luggage and give me a receipt. (I do get to keep my bag, in which I've learned to carry one night of essentials.
There was no charge for gate checking the bag.
I know it's a safety issue, because people are crazed about over-carrying carry on luggage. If you have to exit the plane in the hurry, you'll be crawling over piles of suitcases that fly everywhere when the overhead compartments burst open.
but the reason people freak out and carry so much crap: the airlines lose luggage.