Whis at fault
I'm a plumber & got a call for a simple job to replace hot water valve that was dripping under sink, so I get there and cut the old valve out and install a new one, job was done ✔️ but hild on, then he asks me to replace another (needle) valve on the cold line also inside kitchen cabinet, which he said was for his fridge line, & is piped in a way that goes under his cabinet and into basement ceiling and outta of sight, soo I agree to change valve for extra charge, so I install it & then head down to basement to turn the house water on, as im coming back up the stairs & head into the kitchen i see the home owner bent down inside the kitchen cabinet turning all the valves on, am like in my head what the f#@% are you doing, anyways I done, get paid. I leave to the next job, but the later that night he calls me saying his basement flooded, turns out the needle valve i replaced wasn't for the fridge but for an old reverse osmosis filter system, someone previous disconnected it but left an open water line inside ceiling without capping it, you can guess the rest