Donald Trump 's looking out for the White House Press Corps ... warning one photog to watch his camera -- 'cause it got pretty close to shattering a very old mirror.

The President of the United States was chatting with Australian Prime Minister Anthony Albanese at the White House Monday ... when he spoke to one of the camera operators -- telling them to be careful around a mirror in the back. Gallery Launch Gallery Getty

DJT says, "You better watch that. You're not allowed to break that" -- a good-natured barb that got a chuckle out of the room ... and Trump then followed it up by revealing the mirror's not a cheapie from Target. It's actually 400 years old!

President Trump claims he had the mirror brought up from the vaults ... and, one of the first things that happens is a

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