Both the host and the NCIS actor had brief stints on the comedy Laverne & Shirley in the 1970s, with Harmon popping up in 1976 and Leno visiting in 1979, as well as 1983. When Leno thought he'd surprise Harmon with a blast from his acting past, Harmon countered with a clip of his own.
"Just happened to have something?" Harmon said, as Leno threw up the clip.
"I kind of knew you were going to do this, Jay," Harmon said, before pulling out a clip of his own.
"Oh, shut up!" Leno joked.
Check out the clip from Thursday's Tonight Show.
Notable deaths of 2014 [PHOTOS]