You'll have to indulge me because this isn't my screwup, or even my mate's, but the story he told me seems to belong so...
After graduating, my friend got work as a labourer, working for a bloke he soon decided was an idiot and a flake - a posh boy who'd decided his future lay in property renovation. Through his posh dad, he'd got a job to work on a town house in Chelsea - one of those tall, thin, four storey jobs. Boy genius called my mate over and pointed to a wall in the cellar - "Remove that wall; we're going to make this an open space". My mate looked at it rather dubiously, knocked it with his knuckles, and said "Are you sure? That's a supporting wall isn't it?" PB gave him the kind of indignant look you'd give the help if it was getting lippy, and ordered work to commence. Oh well, thought my mate, gabbing the sledgehammer, in for a penny....
He'd got the hole up to about six or seven feet across when suddenly there was a bang like a bomb blast, and dust everywhere, with lots of groaning. "I was up the stairs in about a second and a half." Turned out the floor immediately above the breached wall had sunk in the middle, so the entire floor was now a sort of v-shape. As was the floor above...and the floor above that one...and the floor above that.