The dishwasher knockout plug should have been removed. New disposals ship with the dishwasher inlet sealed by a moulded plug, which must be knocked out before the dishwasher hose is connected. How it shows up later: the dishwasher backs up, sits full of water, drains into the sink, or overflows — and the disposal gets blamed. It's the single most common installation error on this appliance, and it happens whenever a unit is fitted by someone who doesn't do it regularly. Worth knowing: if your dishwasher started misbehaving after a disposal was fitted, this is the first thing to check and it takes seconds.