An isolation valve that will not close is usually the slot
I went to change a basin washer in April and turned the little valve under the sink. The screwdriver slipped. I turned it again, harder. The slot rounded into a shiny oval. I thought the valve had shut because I had twisted it as far as it would go. I took the tap head off. Water came up. The valve had not closed. I had polished the slot and left the ball where it was. I put the head back, found a better driver, and stopped. I could not isolate. I did not go on.
An isolation valve sits on the supply so you can shut one tap without killing the house. The slot in the end tells you where the ball is. In line with the pipe is open. Across the pipe is shut. A small screw sometimes holds a cap on. The slot is the handle. If you cam it, you are not turning the valve. You are carving a hole. People blame a seized valve. The valve might still work. The slot does not.
A tap that drips is usually the washer in the head is why I was under there. I check the isolation before I take the head off, every time. I turn the slot to shut, then I open the tap. If it still runs, I am not isolated. I do not start. Hose fittings leak at the washer, not the hose is the same order outdoors. Shut it, test it, then take the fitting off. A wet surprise is a slot you did not actually turn.
When I check I use a screwdriver that fills the slot. A thin bit rides up the walls and takes the corners off. I look at the line. I want a clear across, not a maybe. I open the tap and wait. A dying trickle can be what was in the spout. A steady run is a valve that is still open. If the slot spins without the line moving, the slot has sheared from the mechanism. That valve will not close. I stop.
I do not re-pipe a bathroom because a slot is oval. I do not force a rounded slot with grips until I snap the body. If I cannot isolate that tap, I stop. The house stopcock is a different job, and only if I know it and I can get to it. If I cannot, I leave the tap dripping and I wait for someone who can shut the water.
Skip a coin in the slot. You will round it. Skip turning until your wrist hurts. Skip assuming shut because you have always turned it that way. Look at the line, then test the tap. Skip taking the tap apart on a valve you have not tested. Skip PTFE on the slot. It is not a thread.
Some valves have a tiny screw in the middle of the cap. That screw is not the shut-off. I have seen people tighten it and think they have isolated. They have pinched a cover. The slot underneath is still open. Take the cap off if you must, use the slot, test the tap.
Night drips and Saturday jobs are when this shows. You are in a hurry and the basin is the only one. I now test isolate the evening before I buy a washer, not with the head in my hand and the water coming.
I do not need a new basin because the valve would not shut. I need a slot I can turn, a test that the tap goes dry, and a stop if it will not.
If I later add a buy link here, it will be a paid promo.