Burst pipes in the loft, when I say pipes I mean leaks in the plural. It was a very old house which we thought was well lagged but not well enough. Water was literally pouring through the ceiling in five different places, buckets couldn’t catch enough, I had to use plastic under bed boxes . I will never, ever forget it. It only stopped when we turned the water off at the mains , it took two days for the plumbers to fix all the holes.
We drained the system down when we left for the winter after that, including putting compressed air through to dry it out totally.
I wish I hadn’t remembered that!