One of the most stressful parts of flying is knowing that your ability to board your plane and thus reach your destination on time is dependent on the whims of whoever happens to be staffing the flight crew and security checkpoints that day.
While some people (and particularly women) have found themselves removed from a plane because someone didn't like what they were wearing, others have found that anything from how they're holding their children to the genetic conditions they have has threatened their voyages.
And those who would cluck at the passengers involved for not keeping abreast of the airline's policies may not understand that those rules can vary wildly in how staff members interpret them.
Furthermore, one mom recently discovered that travelers have little recourse if the TSA agents they deal with are ignorant of both those policies and of human biology.