Watch Conan O'Brien Mock Jay Leno on the 'Late Show'

There's one rule in the world of television worth following: don't steal a man's TV show. He won't forget it.

You may remember, back in late 2009 and early 2010, how Jay Leno attempted to take "The Tonight Show" back from newly promoted host Conan O'Brien, who had been in the position for only about half a year. Conan was understandably pretty upset, but less so after he departed with about $35 million in severance pay and landed a new show on TBS, "Conan."

But the bad blood remains, as Conan appeared on "The Late Show with David Letterman" and proceeded to mock Leno during his conversation with Letterman. "You know he's watching right now," said Conan of Leno. "He's getting a live feed in a sattelite truck right now." He followed that up with his pretty spot-on squeaky Leno impression.

Of the two, Letterman was much less upset about Conan having lost his job. The continuing gag between the two was how thrilled Letterman was over the disaster over at NBC and his subsequent advantage in the ratings. In fact, Letterman called it "the Golden Age" of television," which elicited a laugh from Conan.

Check out the video of Conan mocking Jay Leno below: