In 1968, Robert Vaughn and David McCallum were originally slated to reprise their roles as Napoleon Solo and Illya Kuryakin in the second attempt at a pilot for "Yellowjacket Greenapple," the episode entitled "The Prisoner's Dilemna." To avoid certain copyright complications, Vaughn actually played the role of Kuryakin, taking on the character's name and characterization (if not his accent), and McCallum played a Brit-accented Solo. Their scenes were all set in the same cell, and consisted of them furiously and constantly double-crossing each other, with no explanation as to what their presence had to do with the main plot, which dealt with Greenapple trying to infiltrate a mental institution. Fans have speculated that Greenapple was unsuccessfully trying to contact them, but the episode ended before his success or failure was shown. Later episodes did not revisit the scenario.
In 1973, Robert Vaughn was invited back to the set of YGPI in order to reprise his MFU role as Napoleon Solo, something he would not do again until 1980, in the MFU film "The Fifteen Years Later Affair." In the episode, entitled "The Marinetti Simulacra," the U.N.C.L.E. organization was portrayed as an organization dedicated to fostering world peace through the stalemating of nuclear proliferation. Solo, now its head (also wearing an eyepatch, a recurring motif in YGPI), was the antagonist of the episode, attempting to distract and/or thwart Greenapple from his assignment of detonating a small nuclear device in an endless, Levittownesque New York suburb. Due to a personal dispute with the director, Vincenzo Malatesta, over the aesthetic properties of the Vietnam War, all of Vaughn's dialogue was overdubbed with the ominous grunts of a wild boar.
I found these tidbits originally posted as a reply to someone's question on a "Man from UNCLE" fan community. The members didn't seem to appreciate the information very much, unfortunately.