SWG.
A puck feeder adds CYA to the water, which means you need to maintain a higher FC level, so you turn up the puck feeder which adds even more CYA to the water, which means you need to maintain a higher FC level, so you turn up the puck feeder which adds even more CYA to the water, which means you need to maintain a higher FC level, so you turn up the puck feeder which adds even more CYA to the water and on and on until you can't add enough FC fast enough and you get algae and the CYA level is so high that the chlorine level you need is beyond the ability of any test kit to measure. And then you have to empty the pool and start over. Except you have algae residue everywhere this time.