I will admit, I was one of the skeptical ones reading the first post. I was lucky enough to tour backstage at Fantasmic as part of a class during my CP last year, so I got to see how that change worked. But reading further, I realized that with the center lift not working, they would've had to go with one of their many contingency plans for that, so it would make sense to consolidate the change to one performer with a much simpler costume piece (though now I'm super curious about how they got the cauldron out there, can you tell I'm a theatre tech?). I'm glad I waited until I reached the end of this to respond, and glad you were able to find that video! I've seen a few contingencies employed at Fant, mostly with Vivian the dragon being her cranky self and having Maleficent have to battle Mickey the whole sequence, but the number of things that seemed to go wrong with your show was a crazy occurrence!
I've been going to Disney for years, was a CM for a little over a year and am still in Orlando with an AP, so I've seen my share of misfires, both show and ride. Some of my favorites have to be:
-In the Enchanted Tiki Room one evening, as the storm started, the sound system gave a little noise like when a Macbook encounters an error and gives a pop-up message, and then all of the sound cut out and the exit doors opened for guests to leave, all while the animation of the birds continued. Almost a little creepy to see them move with only the sounds of clicking mechanisms, lol.
-A few friends and I rode Under the Sea: Journey of the Little Mermaid one night, and something had gone wrong with the Ariel animatronic in the Under the Sea scene. Her head had gotten stuck at an odd sideways angle outside of its usual range of motion, and with the realistic skin twisted with it, it looked like someone had snapped the poor princess's neck

. It sounds horrifying, but it was also so funny we cried laughing through the rest of the ride. Luckily, it was super late at night, so not many other people or kids saw it!
-New Years Eve Eve 2014, a few CM friends and I watched the "New Years" fireworks from the top of the Contemporary, and I snapped a great pic of one of the perimeter fireworks on the road just past Contemporary going off without actually leaving the ground first. Good reminder of why they close the road past the resort for those!
-Back in 2011, one of my best friends came with my family on a trip. Since it was her first time, we slogged through a super heavy thunderstorm at EPCOT one night, waiting for Illuminations to finally get a window to go. They finally got the go, and at the part where the globe emerges, it wasn't lit up like it was supposed to be. I could see faintly a couple of guys running about on it as it was moving into position, and they managed to fix it not too far into it's solo bit, but I was waiting with baited breath to see if we were just going to get several minutes of a spinning dark orb and some fountains

-And of course there's always the infamous Messtival of Fantasy. I was working at The Pirates League in Adventureland that day, so I didn't get to see it for myself, but we sure heard about it from guests for the rest of our day, and a bunch of us watched the video of it on break the next day. That was painful to watch; so so many things gone wrong, and no way to just down a whole parade and get it off-stage. Mad props to the performers that day, they kept it together so well.
Sorry for the long post; it got away from me! I'm a theatre tech, so I love a good tech-gone-wrong story, so long as everyone emerges safely!