When is it correct to salute with your left hand?

When required as a civilian to render military courtesy but you still want to send a message to the Soviets? ;) An explanation is probably required for that one. I was a civilian/contractor/technical specialist in West Berlin. Under the SOFA, I was there under orders and as a member of the Berlin Brigade. When driving to and from West Germany, we would pass through Checkpoints Alpha and Bravo (lesser known than the more famous Checkpoint Charlie), and there was a corresponding Soviet checkpoint to process through. At the Soviet checkpoint, the driver would exit the vehicle, approach a Soviet sentry on duty there, receive a salute, and respond. (We were told that they were all GRU) After he examined our travel documents, he would return them, exchange salutes again, and we would then enter a building for processing of those documents. Once that was finished, we would return to the sentry, repeat the whole process, and drive off - only to repeat the process at the other end of the corridor. I conveniently "forgot" all my training, and always saluted with my left hand.

When I was recovering from shoulder surgery and my right arm was in a sling, I simply gave a verbal greeting. I was stopped once for not saluting, but it was not because I failed to salute with my left hand. It was because the person altogether missed the fact that my right arm was in a sling and expected a right-hand salute. More of a lack of his situational awareness than anything...

When Piping The Side as Honors' Boatswain's Mate and you pipe with your right hand - you are permitted to salute with your left. Something I've done on more than one occasion.

When carrying a rifle at right shoulder arms. A rifle salute is executed with the left hand.

When you're Bob Dole.

When your right hand or arm is missing.

When your right hand is busy. Don't ask.

I don't actually know. I know when I was new I had my hands full and saluted with my left out of reflex. Got chewed out for it and was told next time just not to salute.

If you are opening the door for the President and you are the Marine holding the door with you right hand.

Although saluting is allowed with the left hand when the right hand is either missing or injured, it is dependent on the installation's commander. While an outpatient at WRAMC (before its move to Bethesda) with a right arm injury I looked it up. That week, I walked past a LTC and gave him a good left hand salute (better to be safe than sorry). Made it less than 2m before he stopped me asking why I saluted left handed. According to this LTC, left handed saluting is at the installation commander's discretion and it was not done at WRAMC. Ironically, 3 days later I took a good 25 minute ass-chewing from a female butter-bar for only rendering customary greetings and not saluting. It should be noted that in both instances my right arm was in a brace and slung.