WASHINGTON — There was Robert F. Kennedy’s still-mysterious phone call with an Izvestia correspondent, actually a Soviet spy, on Dec. 1, 1960, signaling that his brother, the president-elect, wanted to change the nature of the United States’ relationship with its Cold War adversary. It wasn’t exactly a success: First came the Bay of Pigs, then the Cuban missile crisis.

There was Richard M. Nixon’s secret channel to the South Vietnamese through Anna Chennault, a prominent Republican fund-raiser, urging the South Vietnamese to deflect President Lyndon B. Johnson’s effort to join peace talks in Paris because Nixon, she said, would give them a better deal. Fifty years later, historians are still arguing over what Nixon’s direct role was, and whether, as Johnson railed, the action was “treasonous.”

Back channels during presidential transitions are not unprecedented, but they are always fraught, as President Trump and his son-in-law, Jared Kushner, have discovered in recent weeks.

In the end, the trouble hinges entirely on the content. “Getting-to-know-you is fine,” said James R. Clapper Jr., the director of national intelligence under President Barack Obama who raised the alarm when he saw intercepts suggesting a series of contacts between the Trump transition team and Russians. The risk, he said, comes when those doing the talking violate “the tradition of one president at a time.”