f you aren’t at the boarding gate when your flight is called, you probably aren’t going to get overhead bin space. If the flight is full, and you aren’t among the first half of passengers on the plane, your carry-on bag is likely going to be taken away from you and gate checked.
Half the time when you get onto the plane, though, you still see overhead bins with plenty of space. You were lied to! The bins weren’t full after all. So what gives?