On the first show, Steve opened with "This show is going to go on forever." He was referring to the show's length (at the time) of an hour and forty minutes, but many TV people like to joke that Allen was psychic.
Jack Paar (1957-62)
Jack quit on-air during an episode because NBC disliked a joke he made using the abbreviation "WC", which can stand for "Water Closet" (bathroom) or "Wayside Chapel", to the point of airing a news report over said joke. Before leaving, Jack said "I am leaving The Tonight Show. There must be a better way of, uh, making a living than this." His announcer Hugh Downs was forced to finish the show himself.
A month later, Jack returned with the explanation "As I was saying before I was interrupted... When I walked off, I said there must be a better way of making a living. Well, I've looked... and there isn't."
Johnny Carson (1962-92)
1973: Johnny had invited Uri Gellar on the show to demonstrate his "psychic spoon-bending" abilities he was promoting at the time. Carson, a former magician, suspected (rightly) that Gellar was making use of simple stage tricks, and with help from James "The Amazing" Randi set up a test which showed Gellar unable to bend a single spoon in front of the camera.
Early 1970s: During a period where the show was still doing live commercials, announcer Ed McMahon was doing one for Alpo dog food which involved him sitting in a chair, pitching the product while feeding a small dog on the floor...but the dog wasn't hungry and wandered away. The camera went to a tight shot on Ed's face for an instant before pulling back to show that Johnny had come on-set on all fours, barking and (pretending to be?) eating the dog food himself!
Jay Leno (1992-2009/2010-2014)
May 29, 2009: On his Grand Finale last show before a small hiatus, Leno revealed all the kids who were born to crew members during the show's run. "And that's what I would like my legacy to be: when these kids grow up and say "Hey mom, dad, where did you guys meet?", I want them to say "We met on the set of The Tonight Show"."
Conan O'Brien (2009-10)
June 1, 2009: As strange as it might be for what's effectively a series premiere to have an MOA, Conan's first show began with the NBC "In Living Color" Peacock and voiceover followed by a sketch of Conan sitting in his New York apartment, going over everything he needed to do to prepare for the show, and realizing he forgot to move to Los Angeles. Unable to hail a cab, he proceeded to sprint across the entire nation in a single morning, while "Surrender" by Cheap Trick played in the background.
And when he got there, he realized he left the studio keys in New York...so he drove through the wall with a bulldozer.
January 20-22, 2010: With his time on the show running out, Conan announced that he'd be exploiting a loophole in his contract to do whatever he wants on NBC's dime.
January 20: Blowing $1,500,000 dressing up the world's most expensive car as a mouse and playing the original master recording of The Rolling Stones' "Satisfaction".
January 21: Followed up by buying a Jockey and a 2009 Kentucky Derby winning race horse wearing a mink Snuggie watching restricted NFL footage. Total cost: $4,800,000.
January 22: On his final episode, he bought an original fossil of a Giant Ground Sloth from the Smithsonian, which was spraying beluga caviar on an original Picasso painting. Total Cost: $65,000,000!
Too bad Conan had to admit it's not real. Although all of the products he displayed during his "New Tonight Show Mascot" segments were rented or on loan from other organizations, the music certainly wasn't. According to comments made by Roots drummer Questlove (on his Twitter page), many of the tracks Conan used during his final week were on NBC's banned/restricted music list because the rights to clear them were through the roof. For instance, the music that played when Tom Hanks appeared on Conan's finale (the Beatles' "Lovely Rita", in tribute to Hanks' wife) costs $500,000 in clearance rights. Yes, Conan really did stick it to NBC in his final week. And it was awesome.
January 21, 2010: Pee-Wee Herman's explanation of the late-night debacle. Although beginning normally, with an adorable stuffed giraffe ("That's you, Conan") and NBC Peacock, it escalated into the most awesome thing ever — Conan's attorney, according to Pee-Wee, is OPTIMUS PRIME.
January 22, 2010: Conan's final speech. All throughout the week, he had made several escalating jabs at NBC management, and everyone expected him to end his tenure with some snarky commentary on the way the show was handled. Instead, he ended with this after thanking NBC:
"I hate cynicism. For the record, it's my least favorite quality. It doesn't lead anywhere. Nobody in life gets exactly what they thought they were going to get. But if you work really hard and you're kind, amazing things will happen. I'm telling you. Amazing things will happen."
January 22, 2010: The very last moments of Conan's tenure — himself (on guitar), Will Ferrell, Billy Gibbons (of ZZ Top), Beck, Ben Harper, and the Tonight Show Band playing a rendition of "Freebird" in front of a giant American flag. Ferrell even played the cowbell at one point. It was more awesome than the goddamn Batman.
Jimmy Fallon (2014-)
February 17, 2014: On his first night, he allowed Joan Rivers to appear after Johnny Carson (and out of respect for Carson, Jay Leno) banned her for a quarter of a century because of a grudge.
March 21, 2014: Jimmy decided to decree that dancing was illegal on the Tonight Show. His guest was Kevin Bacon. Mr. Bacon was not amused. As such, he decided to express his displeasure in a very familiarstyle.