It would have to be when we went to Hawaii for my sister's wedding. We were all going - my parents, grandparents, family friend - all together. We had a very early flight out to Dallas, where we would connect for Hawaii. We were delayed a little, so that conenction was tight! We did make it though - whew! But as we are going, halfway across the water we actually hear the old, "If there is a doctor or nurse on board the aircraft, please identify yourself to a member of the flight crew." Yup, that's real! Okay, someone was having some issues. We had to turn back to San Diego, where paramedics boarded the plane and removed the person in distress. Nobody was super happy about it, but, okay, medical emergencies happen.
However there had been another issue on this flight - one of the lavatories was out of service. As we made our way back out over the Pacific, they had closed all lavatories on one side of the aircraft - then the other. There was a widespread issue with the septic system. Apparently, they legally can't fly without a working restroom. They needed to turn us around to a maintenance hub in LA!

Again, we're heading back to the mainland. If you had taken a poll, we'd have voted tht we could hold it, but they needed the plane to be somewhere it could be fixed. At this point it's already like midnight home-time and we got to the airport at like 5 AM. Here's where it gets really bad though. They took all the passengers off that 747 widebody aircraft with plenty of extra space and crammed us into a 737!
So, we're finally on our way again - we don't get to Hawaii until like 1 AM local time, which is like 5 AM home time, almost 24 hours of travel! We were amazingly still able to pick up our rental car, but we are all tired and crabby. We drove to the wrong hotel, then had to find the right one. I thought I was going to die! The next day we spent in Honolulu so my sister could do some shopping for the wedding. It's just a big, dirty city and we were all pretty snippy with each other. It was a rough start, but things did get better once we got over to Kauai, which really is a paradise. Once we could relax, things went much better, but man, it was the trip from hell!