1.) Airlines suggest minimum 2 hours before departure for a reason. There are long lines at the airport! Bag cut-off is an hour in many airports.
2.) Getting though TSA, again, long lines.
3.) Not enough connecting time. When you book a flight with a connection, make sure you have at least one hour to make that connection. More if you are on a commuter flight and have to gate check a bag and wait for it on the other end. That takes more time. If you are going to a destination with only 1 flight a day, give yourself ample time.
4.) At the wrong gate! Every day, all day! Make sure you are at the right gate,
they change often!!! Do not wait until it is departure time to ask!
5.) At a restaurant or waiting in line for coffee.
The 2 things you CAN do if you've missed your flight:
1.) Get booked for the next flight.
2.) Go stand-by for the next flight, Maybe you will get on?
Enjoy the journey!