David Letterman Makes Rare, Long-In-Coming Visit To ‘Tonight’, Trades Quips And Slaps With Jimmy Fallon

John Oliver’s not the only talk show host who can drop in on another show unexpectedly. Last night, the great David Letterman made a rare appearance on the Tonight Show right in the middle of Jimmy Fallon’s monologue. Catching the audience – if not Fallon – by surprise, Letterman dropped by to see his old