[Butch Cassidy Slept Here]

Those people you see carrying-on bags that should have been checked will get nailed (for bag fees) if they are connecting to a flight which uses regional jet, or commuter prop, equipment. Otherwise, yes, they may get away with it. It's a toss of the dice. Some gate agents and flight attendants will let you carry on the kitchen sink. Others will nail you if you're one millimeter over the allowable dimensions.