Periodically (for no apparent reason), the water supply valve opens and "tops off" the quantity in the water closet. Can anyone tell me why this happens, and how (other than installing a new water level control) I can fix it? Many thanks, . . . Helper56
Most likely, the fill-valve is doing what it is supposed to do, keep the tank water level at the proper height. I am guessing that you have a leak through the flapper. When the water level in the tank reaches a certain point, the fill valve replenishes the water. Try adding some food coloring to the tanks water and watch the bowl for an hour. If the water in the bowl changes color, you need to clean or replace the flapper. May also need a chain length adjustment. The chain only needs to allow the flapper to "just" seal.
If you have never made a mistake, you haven't done much.