Didn't actually witness this one, but it happened to my father's coworker many years ago, and it must have been classic...
Apparently, 'Joe' had been drinking throughout the day and the family went back to MK for some rides. The happy family (Joe, Mrs, two kids) got on Pirates. Now since Joe was a bit uninhibited from his imbibing, at one point on the journey, he grabbed his son's sword, jumped out of the boat and started to battle with the pirates. Needless to say, ride stops, security comes, escorts Joe out of the ride and out of the park. From what I understand the resulting argument could be heard all up and down the eastern seaboard when Mrs. Joe caught up with him. And there was no kiss and make up either - she went and hired Mr. Lawyer and Joe kissed half his assets goodbye....
The only meltdown I saw was one evening in the MK. Being a meticulous planner, I had all my ADRs laid out and my friends and I would meet for our appointed reservation times. We all knew to meet at the restaurant and we were waiting patiently for our table when a harried wife, husband and small children entered Liberty Tree Tavern. I don't know if the wife thought she had an ADR, or didn't know she needed an ADR, but the CM at the desk told her that they were fully booked and couldn't guarantee anything. Well, hubby and wife start to go at it. I guess they were all tired and hungry. Words were exchanged. Dad took kids outside, and mom burst into tears. Part of me wanted to walk up and tell them to take our reservations...I hate tension of any kind....apparently the mom's tears had a healing effect and the helpful CM said, 'let me see what I can do'. Not sure how the story actually ended as we got seated at that moment...but seeing dad storm off, mom in tears and no one happy was sad....
I have one more, not really a meltdown, but one of my favorite stories from one of my friends. She and her bf at time (husband now) were at WDW for a vacation. Rich wanted to take a photo of Loni by one of the icons and had her pose. He wasn't happy with the pose and kept saying 'turn more left, move your hand, look right....' Finally, Loni lost it and snapped, "Rich, would you take the F%$^ing picture?" Rich looked at her with his mouth wide open and said, 'There's no cursing in Disney!' He took the photo, they laughed, and it's still a great story all these years later.