If a door between an already fully flooded compartment (From damage or other non spawned means) and an unflooded compartment is opened, water will not fill the unflooded compartment at the expected rate if at all through the door. Quite often nothing will happen when it should be flooding.
Example scenario: A Ship has sustained critical damage, sank and is sitting a good 10ish meters below the surface. Opening a door between a flooded compartment used to and logically should flood the adjacent compartment.