According to MSNBC's Chris Matthews, "in July of 1971, President Richard Nixon signed the 26th Amendment into law, making 18 the [minimum] age to vote." The Hardball host made this pronouncement as he introduced a segment attacking new legislation in North Carolina which liberal activists charge violates that Amendment.
But of course, it's patently false that Nixon signed the amendment "into law." Indeed, no president signs any ratified amendment into law. Ratified amendments take effect immediately either "when ratified by the legislatures of three fourths of the several states, or by conventions in three fourths thereof," depending on which manner the Congress proposed when introducing the amendment. What Nixon did on July 5, 1971 was witness the certification by the administration of the General Services Administration that the requisite three-fourths of the nation's state legislatures had ratified the Amendment. From Nixon's remarks that day, via the American Presidency Project (emphasis mine; YouTube video below page break):