Like others, my biggest brewing disasters happen not on a brew day, but somewhere along the line. To wit:
A couple of years ago we had a small group (8-10) friends over for a summer barbecue. Everyone was sitting out on our deck. Underneath the deck and out is my brewing/ meat smoking area, with my brew room in the basement.
I went down to pull some meat out of the smoker, and took it inside to carve in my brewing room. I accidentally dropped my by heavy set of tongs during the process. Ordinarily no big deal, except they landed and karoomed off a 5 gallon glass carboy that currently was serving as a secondary fermentor for a 10% abv Russian Imperial Stout.
The tongs hit the edge of the carboy where the vertical sides start curving up to the neck. This broke a very small hole at the spot of impact, causing a small stream of RIS to squirt out of it. I though that if I lifted it onto the table and started a siphon into a keg or bucket, I could capture and save most of the RIS.
Unfortunately, the small hole marked the end of the structural stability of the carboy. When I attempted to pick it up, it failed, and RIS poured all over the basement floor. Fortunately, the carboy fell apart into large shards, which I easily picked up and placed into a trash can. I then quickly mopped up and hosed down the RIS mess (my basement floor has a floor drain, which greatly helps when such disasters occur), and I had every thing cleaned up and done in 10 minutes, before a guest could come downstairs to fill a beer out of my kegerator and see what I have done.
I carved the brisket, and headed up to the deck for dinner. No one knew of my disaster. I told my wife what had happened after the guests went home.