Two things come to mind about the basement darkroom I set up from scratch.

First, I made the mistake of building dry-side storage shelves with "kick bases" that do not allow cleaning/mopping under them. I have no floor drain to collect spilled fluids (WILL occur at some time). Since one of my minor water connections started leaking - fortunately I was home to stop it - I've made the necessary changes to that joint, but I was also fortunate in having covered the entire concrete floor with the type of thick rubber anti-fatigue mat interlinking sections that have a bunch of one-inch holes in them, so these holes held most of the leaked water. I now keep a bucket and mop handy.

Second, I wish I had not placed all the plumbing lines inside the walls, which would mean tearing the walls apart to locate/repair any leaks.