Another in a litany of crazy repairs was a recently installed, and expensive, wall mounted tub filler. It seemed the tub filler didn’t want to fill the tub. The usual suspects were investigated. Was it the anti scald valve that has a prescreen and acts as somewhat of a flow restricter? No, totally clear. Was it the aerator on the faucet? Also clean. Was there an obstruction in the roughin valve or spout? No again. Faulty cartridge? it was pulled out and examined and looks good. At this point there are no options left to investigate until the savvy plumber pulls out another internal component with a wrench and finds a third screen with particulate behind it. After reassembly the tub filler now fills.
A piece of sawdust and a tiny scrap of plumbers tape found their way to gum up the inner workings of the tub filler. How did this stuff get in there, nobody knows.