The worst one I had was last year for my Study Abroad to Costa Rica. We flew from our airport to Atlanta, which went just fine. When we got to Atlanta, we found the terminal and everything pretty quickly, and went to find food quick before we left again. And then we waited to board, and waited, and waited... eventually we just went to the bar with our professor because why not, we were stuck in the airport anyway. We ended up waiting 5 hours to board because, according to the ladies working the boarding gate, that the first pilot we had was over his flight hours, and couldn't take us. The second that they called had to drive like half an hour to the airport, and it turns out he didn't have the certification to land in San Jose because of the mountains. The third ended up driving an hour to the airport and THAT'S when we finally got to leave. We finally landed in San Jose somewhere after 2-3 AM, I believe (I was too tired to remember really).
The trip was really fun, but it was a pain again on the way back. Once again we got stuck in Atlanta, this time in customs. To make it short, a few of us nearly missed our flight completely (and would've been forced to stay overnight). I ran across two terminals and made my flight by barely 30 seconds. Never going through Atlanta's international terminal again if I can help it.