The type of thing that would be breaking news were President Obama a Republican. Not saying it should be, just saying it would. Clearly he got into Marine One and somebody pointed it out to him. Once he was aware of the gaffe, he probably handled it about as well as he could. Going back and saluting the Marine would have been awkward, to say the least.
What's hard to imagine here is that President Obama has been president for more than four years. Hasn't this become routine? Related exit question: Has the President used the word corpsman since his "corpse-man" mispronunciation was pointed out?