I was flying on Airtran about 15 years ago and connecting in ATL. I used to smoke then so I calmly took the inter-terminal train to get to Terminal D, IIRC, smoked my cigarette and made my way to the departure gate. I could have sworn I had plenty of time but needless to say my plane had departed already. Thankfully there were seats available on the next flight and I got on without trouble.

More recently, on a trip from EWR to BNA, I and a colleague were stuck in a ridiculously long security line in Terminal A. My colleague thought we were cutting it too close so we started begging pax in front of us to let us through. I thought we had plenty of time - about 25 mins to departure - but I went along. We went through security and he headed straight to the gate but I stopped to go to the restroom. When I reached the gate, maybe 2 minutes behind him, the gate was shut already. No amount of pleading with the gate agent helped. I had to then sheepishly call my colleague and have him move our client meeting to later in the afternoon.