Originally Posted by aerobod

The only airline I know that enforces carry-on at the gate well is Ryanair. They arrange everyone inside a bull-pen before the gate to the aircraft is open, then an agent walks through the bullpen with a plastic box with a handle on it that must fit over your allowed carry-on, they also check and mark your boarding pass. Once they reach the end of the bull-pen, the flight is closed off and the gate to the aircraft is opened. A structured approach with no leniency, but very effective.