What's inside your carry on bag? You've got light scooby snacks and water (post-security purchase or refilled water bottle) packed - just in case you get stuck on the plane longer than expected. You remembered to pack toiletries - to freshen up during long layovers. And surely you packed a change of clothes - in the event you arrive in a city but your checked bags don't. What else needs to go in the checked back? More essentials! House keys; mobile phone, and important documents such as hotel/car rental information.
Common sense? Yes. But, sometimes travel plays tricks with common sense. For instance, I just returned from a flight where a woman was rerouted to Dallas, instead of Los Angeles, due to a missed connection. She will make it home tonight, but her house keys - which she later realized were stored inside her luggage, will not. Lesson learned: Think through all the important items you might need in your carry-on luggage, both during regular and irregular flight operations, and pack accordingly.