Kitchen sink
The kitchen stop is there so you do not flood the stack.
A failed angle stop or a loose coupling under the sink is a common, boring leak. It becomes expensive when it runs overnight into particle board and the ceiling below.
Close both stops under the sink if they turn by hand.
Leave a note on the sink. Do not keep the drip ‘just for the dishwasher’ if they share a supply.
Open the cabinet and look with a light.
Swollen sides, rusted stops, and a wet floor tray tell you more than the countertop stain.
Particle board hides volume.
The cabinet can hold a surprising amount of water before it shows downstairs.
Questions people ask
One stop is seized. What now?
Do not force it. Isolate farther upstream with building help, or call.
Can the dishwasher still run?
Not if it shares the wet supply. Isolate first.