Bing Crosby and David Bowie really did sing "The Little Drummer Boy" together in 1977. Bing Crosby died before it ever aired.
Bing is said to have told a reporter after the taping that Bowie was "a clean cut kid and a real fine asset to the show. He sings well, has a great voice and reads line well. He could be a good actor if he wanted."
Merry Christmas, Bing, wherever you are.