The flapper (or flush valve seal). Its job: seal the tank against the flush valve between flushes, and lift out of the way when you flush. How it fails: the rubber hardens, warps, or picks up mineral deposits and stops sealing. It doesn't fail suddenly — it starts passing water slowly, and the fill valve tops the tank up periodically to compensate. What you'll notice: the "phantom flush" — a toilet refilling by itself with nobody near it — or a faint continuous trickle into the bowl. This is the single most common cause of an unexplained water bill in any property, and it's silent for months. Expect it: to be the first thing to go, and to go sooner if you use chlorine-based in-tank cleaning tablets, which degrade it steadily.