A running or silently leaking toilet. Cost: money, continuously. Damage: none. What's happening: water passes the flapper into the bowl and the fill valve tops the tank up to compensate. It's silent, it's constant, and it's the most common cause of an unexplained water bill in any property. What a week costs you: a meaningful volume, and it doesn't slow down. Nothing is being damaged, which is exactly why it gets tolerated for months. The good news: you can stop the cost immediately without a plumber. Close the angle stop behind the toilet and the waste stops that second. The repair is a cheap part and a standard appointment. Confirm it: a few drops of food colouring in the tank, thirty minutes without flushing, then look in the bowl. Colour in the bowl means the flapper is passing water.