Jay Leno returned to "The Tonight Show" Nov. 7, 2014...

Jay Leno returned to "The Tonight Show" Nov. 7, 2014 -- a night that will live in late night TV infamy. Credit: NBC / Mitchell Haaseth

Jay Leno performed at the Tilles Center Saturday night, and for all I know, Jay rocked the house. But what happened Friday on "The Tonight Show"?

His first real return to "Tonight" since February, the first return of a "Tonight" host to the stage of his renown in anyone's memory -- maybe ever. (Did Jack Paar come back to Johnny's show for a brief curtain call? Some sharp-eyed readers have reminded me that indeed he did -- in 1986, in at least one instance.)

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Jay Leno performed at the Tilles Center Saturday night, and for all I know, Jay rocked the house. But what happened Friday on "The Tonight Show"?

His first real return to "Tonight" since February, the first return of a "Tonight" host to the stage of his renown in anyone's memory -- maybe ever. (Did Jack Paar come back to Johnny's show for a brief curtain call? Some sharp-eyed readers have reminded me that indeed he did -- in 1986, in at least one instance.)

The moment was, to be clear, historic, or at minimum, highly noteworthy.

But in hindsight, the return of this particular king established, if this ever needed establishing, that you can't go home again. Once departed, stay departed. It's over. Move on. Say nice things about the new host if you must -- fine -- but don't go on the new host's show.

Johnny Carson had it exactly right. Go quietly, but definitely go.

Leno apparently did not get that memo, not that one was ever issued nor ever needed to be. Going back was a mistake.

What were the problems? He opened with a monologue that actually began with a joke about "Bangkok." Seriously. I thought there was a law, perhaps unwritten, that said no standup anywhere at anytime must ever make fun of the word "Bangkok" -- unless you're just starting out, say, in the sixth grade and really want to impress some fifth graders.

And it actually went down from there: Jokes about flip phones -- flip phones! -- Lawrence Taylor's old arrest (with an accent on OLD -- didn't that take place four years ago?)

Even jokes about "bingeing" and "Depends."

It was very, very sad. 

Here's why: Jay reminded viewers, forcibly so, why this transition took place, had to take place. Fallon, visibly nervous the entire night or at least as long as Leno was within punching distance, probably wanted to throttle the producer -- or more likely NBC executive -- who insisted on this booking.

Just say no, Jimmy. That's your prerogative. Just say no.

Or at least next time.

The interview, such as it was, offered nothing: Just two guys circling one another, looking for laughs that refused to come, trying to establish something -- when nothing could possibly be established.

Jay is the former host. Jimmy is the current host. This combination does not mix. It should not mix. What was NBC thinking? NBC was not thinking. There, that's your answer.

Jay remained on the couch -- or rather chair -- when the next guest came out, and that was Mistake No. 2. Lucy Liu was the guest, and you, as host, know you're really in trouble when a guest has funnier lines than either you or the former host, who happens to be sitting just two feet away.

She noted that if the chat with Jimmy didn't work out "I'll just turn to Jay."

What happened next was almost disturbing. Liu told a story about how she was with some friends from Israel, who (when they saw Jimmy Fallon on the street) mistook him for a construction worker.

Fallon started to laugh, and then Leno took over: A loud, roaring, over the top guffaw, and clearly both a denunciation of and mockery of Fallon's predilection for the loud, over-the-top guffaw.

Poor Jimmy. I almost felt sorry for him.

OK, enough. I didn't start out this fine Sunday morning to write up a mean post about Jay Leno or Jimmy Fallon -- two people I happen to like, and admire. Jay, in fact, is a good guy -- really, unless you subscribe to Conan's view -- and one of the great stand ups in history.

 But Friday night he proved that you simply cannot go back -- you can't return to remind people of what once was, or jarringly remind them that perhaps it should never have been.

The disconnect was simply too great, too -- honestly -- painful.

Let's remember what we should remember: Jay Leno was, for the most part, an industrial strength "Tonight" host who essentially kept the franchise going, and going well, for more than 20 years. He was a marvel of endurance -- a guy who spun hundreds of thousands of jokes, some good, some clearly awful and a few laugh-out-loud funny ones from the day's events over a two-decade span. He had on guests, asked them questions, then said, "Next."

His "Tonight" was a factory line, and the line just kept on going.

But that was then. This is now. Jay's next late night stop will be Craig Ferguson's last show in December. That should actually be a good one. Ferguson knows how to ask questions, and Leno -- if he wants to be thoughtful -- knows how to answer them.

But Friday -- sorry, Jay fans -- was a disaster.