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.

John Oliver wasn't the only late night icon to crash this week. "The Tonight Show" started Monday off with a surprise appearance from the now-retired David Letterman, who poked fun at Jimmy Fallon while reuniting with bandleader Paul Shaffer.