Loss of separation

When the same person or tool does the work, checks the work, and decides whether to go ahead, one mistake sails through every stage because nobody uncoupled from it is left to object.

Your bathroom remodel is almost done. The contractor recommended the scope, hired the plumber, did the work, and this morning he walked you through a completion checklist he wrote, ticking each line himself. He also decides when the final payment is due. You are standing on new tile nodding along, vaguely uneasy but unable to say why, because everything he says sounds reasonable and you have no independent way to know whether the shower pan was sloped correctly under all that grout.

Loss of separation is when the doer, the checker, and the stop button quietly collapse into one component. The tell is simple: ask who could tell you this work was wrong, and the only answer is the person who did it. Unnoticed, it costs you the whole point of review, since a single error now passes three gates at once and surfaces months later as a leak in the ceiling below. The smallest correction is to uncouple one role, not all three. Pay a home inspector who did not do the work to check the wet areas before final payment, and keep the release of the last check as your decision, not his. What changes is that one honest mistake now has somewhere to be caught. This is a false alarm on cheap, visible work, where redoing it costs less than the second opinion.