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Hillary Clinton used Donald Trump's mysterious late-night "covfefe" tweet to take a jab at the Prez and his alleged ties to Russia.

HRC was speaking at the Code Conference in Southern California Wednesday -- primarily about technology and policy -- but couldn't resist weighing in on Trump's Twitter snafu.

Sean Spicer refused to admit Trump's tweet was a typo when he addressed reporters Wednesday, saying ... "The president and a small group of people know exactly what he meant."

Safe to assume Hillary was not part of that group.

5:45 PM PT -- Trump just took a shot at Clinton following her joke about his Twitter gaffe ... on Twitter.