Can you imagine any situation in which you would violate safe gun handling rule #4 (Identify your target and know what lies beyond it)?
Consider the following scenario:
You're at home, asleep. Suddenly, you're awakened by what sounds like someone breaking down your front door. (Your dog, if applicable, is also barking furiously). You hear a crash, then footsteps, then what sounds like a scuffle between Rover and the intruder, then a couple of gunshots and no more barking from Rover. By this time you're fully awake and you've armed yourself and dialed 911. You also shout a warning and rack your shotgun. But instead of hearing a hasty retreat, you hear footsteps pounding up the stairs towards your bedroom. You repeat your warning but whoever's outside jiggles the handle, then starts kicking the locked bedroom door. It looks/sounds like one more good kick will knock it down.
Do you risk shooting through the door?
Note: At no point during this incident have you heard the intruders make any attempt to identify themselves as police.