Vet the subs
I hired a tiler to replace an old fiberglass shower stall with tile and used the plumber he recommended to install the new shower pan (the liner that sits under the tile mortar bed and has vertical sides to hold water in). I trusted the tiler and didn't check the plumber's credentials thoroughly. When water started pouring through the ceiling fixture in the kitchen, a different, reputable plumber discovered that the shower pan had been cut at the corners so it would lie flat.
—Tom Baker, Building Technology Editor