I've seen ONE person in the many years I've been flying get kicked off. He boarded the plane, promptly passed out in the aisle, my doctor dad ran over to figure out what was wrong. His girlfriend was along and told him that they'd just driven straight through the night from a wedding to make it to the plane on time and that the boyfriend was still hungover.  They called an ambulance to come administer care to make sure the guy was ok, but they refused to hold the flight for him. Both were essentially "kicked off" the flight and booked for a later one. Girlfriend was not happy.