Kimmel talked about his hatred for Leno during an interview on Larry King Live. He did a scathing impersonation, complete with prosthetic chin, on his show.
Leno attempted to make peace after that last incident by inviting Kimmel to appear as a guest on The Tonight Show, which he did, but he also used it as an opportunity to shit on Leno a little more.
He saved the worst for a 2013 Playboy interview, where he said this:
"I have a filter mechanism in my head every night when I put together the monologue for our show: If I can imagine Jay Leno telling a joke, then I won't do it, even if it's a good joke."
Harsh! I don't fucking buy it for one second, but harsh nonetheless! Here's the thing: Aside from tracking down funny ads and headlines (something the Internet still loves to do in all sorts of variations), Jay Leno's most "renowned" skit on The Tonight Show was called "Jaywalking." Have a look:
Because it's Jay Leno and your grandparents' Internet abilities haven't expanded to YouTube yet, the video has just over 20,000 views. To give you an idea what kind of company that puts it in, I have a video of a growling dog on my YouTube page that has almost twice as many views (as I'm writing this, anyway). If you've never watched a "Jaywalking" skit and aren't inclined to start today, basically it's a man-on-the-street kind of thing where Leno asks clueless people really simple questions and then basks in the laughs as they fail terribly. It's a really simple premise, and it's been a staple of the Jay Leno comedy show for a long time. Now have a look at this video: