I had just laid a new veneer floor in the bathroom, and was drilling the holes for the door sill. I knew there were water pipes somewhere beneath the work area, so I cleverly decided to drill holes where the previous screws had been located.

Why, oh why, did I drill a 1" hole when the old screws were only 1/2"? I drilled clean through the heating water pipe, ruined the new floor, and brought the ceiling below down for good measure.