One of the problems I've found is when passengers have travelled on on domestic jet (ie SY-BN) and then connect with a regional aircraft (ie BN-HBA). They have carried their 2 bags/items, each at 7kg on the jet and now they what to carry those same items on the smaller regional aircraft. Thats fine on our 36 seat aircraft, but what about the smaller 15-19 seat (Metro/Bandit) aircraft that do not have space for carry-on bags.
This problem is increasing with the increase use of internet bookings. Pasengers don't bother to read what carry baggage allowance they are allowed. In the past there was a good chance the travel agent would tell the passenger about the size of the regional aircraft and it's lack of space for carry-on bags.