A lady friend of ours is now a very angry person, courtesy of Spirit Airlines. Spirit cancelled her daughter's reservation on a flight from DFW to Chicago, claiming that she had arrived at the gate too late.
I went off to consult with the Spirit Airlines Conditions of Carriage. Here is the relevant verbiage:

1.3. Check-In
1.3.1. Check-in begins three (3) hours prior to departure at the Spirit airport ticket
counter or may occur within 24 hours prior to flight departure if eligible for online check-in. Beginning January 24, 2012, customers who choose to have their boarding pass printed by an agent at the airport will be charged a fee at
certain airport locations.
1.3.2. Spirit reserves the right to cancel the reservation or seat assignment of any customer who does not have a boarding pass at least 30 minutes prior to scheduled or posted departure; 45 minutes for Atlanta, Chicago, Dallas/ Ft.
Worth, Denver, Fort Lauderdale, Las Vegas, Los Angeles, Orlando, San Juan; and 60 minutes for all International and U.S.V.I. flights except for Aruba, Armenia, Bogota, Cancun, Medellin, Managua, Panama, San Jose, St. Thomas, U.S.V.I and
St. Maarten which require to check-in 60 minutes prior to scheduled departure time.
1.3.3. Customers wanting to check baggage are required to do so at the ticket counter at least 30 minutes prior to scheduled or posted departure; 45 minutes for Atlanta, Chicago, Dallas/ Ft. Worth, Denver, Fort Lauderdale, Las Vegas, Los
Angeles, Orlando, San Juan; and 60 minutes for all international and U.S.V.I. flights.

My friend's daughter arrived at the gate with hand baggage 45 minutes in advance, but the gate agent claimed that she arrived 44 minutes before scheduled departure time.
This is the first set of conditions of carriage that I have seen that mentions a 45 minute rule for domestic flights, and which also threatens to cancel reservations.
The daughter is now booked on US Airways for tomorrow. Spirit has lost a customer for ever. Not only that, but they have shown enough to never get any of our business. There is an old saying "rules are for interpretation by the wise, and slavish observance by the foolish". The way that Spirit Airlines behaved today shows which category they belong in. I personally have a deep aversion to giving money to fools.