The main thing Jimmy Kimmel did to Jay Leno that caused a big late‑night feud was a sharp, very public joke on Leno’s own show in 2010, where Kimmel mocked Leno’s handling of the “Tonight Show” host change with Conan O’Brien.

What exactly happened

In early 2010, Jay Leno was back hosting “The Tonight Show” after briefly stepping down in 2009 so Conan O’Brien could take over, only to return to the 11:35 PM slot months later after NBC’s messy schedule shake‑up.

During a guest appearance on Leno’s show, Leno asked Kimmel to share his best prank. Kimmel replied:
“I told a guy that five years from now I’m going to give you my show, and then when the five years came, I gave it to him, and then right away I took it back. ”

That was a direct jab at how Leno had handed “The Tonight Show” to Conan — promising to step aside — only to negotiate a return to late night that forced Conan out.

Why it was seen as “humiliating”

Leno later said that segment “humiliated” him on his own show, because he didn’t stop Kimmel or edit those jokes out of the broadcast.

  • Kimmel’s line made Leno look like someone who used a host succession plan as a bait‑and‑switch, damaging his reputation among peers and fans.
  • Leno admitted he trusted Kimmel and let the jokes go uncut, saying, “I allowed it to happen. I didn’t edit it. It was my mistake”.
  • That moment became a symbol of Leno’s fall from respect among other comedians and fueled a years‑long chill between the two hosts.

How others reacted

David Letterman, another rival/peer of Leno’s, famously mocked the whole “Tonight Show” mess on his own show and was delighted when Kimmel delivered that punchline.

  • On Letterman’s “Late Show,” he replayed Kimmel’s joke and joked about Kimmel’s role in the “Jay Leno scandal,” making it clear that many in late‑night saw Leno as the one who broke the unwritten rules of the business.

Aftermath and reconciliation

For years, there was real tension between Leno and Kimmel, but they eventually made peace:

  • Around 2017, Leno reached out to Kimmel after Kimmel’s son had heart surgery, which Kimmel said was a key moment in reconciling.
  • Leno later called the feud “over” and acknowledged that Kimmel’s joke was a deserved response to the messy “Tonight Show” situation, even though it hurt at the time.

So, in short: Kimmel didn’t “do” a physical or violent thing to Leno; he delivered a brilliantly timed, very public joke on Leno’s own show that framed Leno as untrustworthy during the “Tonight Show” controversy, and that moment became a defining insult in late‑night TV history.

Bottom line: Kimmel’s “prank” line during Leno’s 2010 interview is what Jay Leno still refers to as being “humiliated” on his own stage.