Have you ever seen the Home Alone movie series? In one movie, Kevin, the part played by McCaulay Caulkin boarded the wrong plane. In this news story, a toddler was left at the airport in Israel while her family (parents + 4 siblings and 18 suitcases) were in a rush to board a plane for Paris. The toddler was found wandering in the airport by a policeman. The parents only found out their daughter was missing 40 minutes into the flight after being informed by the cabin crew.
A similar incident happened a couple of months ago, that time, it happened to a Filipino family migrating to Canada. That story can be found here.
I cannot imagine this ever happening to us. But of course, we only have two kids. When we were on vacation in Europe last month, we held the kids by their hand, have them walk in front of us and we always check if they're with us all the time.