CLEVELAND — This is why they come. This is why there are 15,000 credentialed journalists here. These conventions are entirely scripted — except when they’re not, catastrophically and deliciously. The political junkies are flying high Thursday morning after Wednesday night’s debacle. Sen. Ted Cruz’s non-endorsement of Donald Trump, and the palpable rage it incited at Quicken Loans Arena, instantly became one of the classic Big Moments in convention history.

Remember Nelson Rockefeller got booed in 1964? No? Well, my colleague David Maraniss does: “Saw it on TV. At the Cow Palace. It was the first emergence of the Western conservative yahoos when they shouted down Rockefeller when he called them extremists.”

My colleague Dan Balz offers another couple of classic convention moments. In 1972, the Democrats gathering in Miami got bogged down in a platform dispute, and nominee George McGovern didn’t get to give his convention speech until 2 in the morning.

And then there was the Jimmy Carter-Teddy Kennedy fiasco in 1980. Carter desperately wanted an arms-raised-together moment with Kennedy, and practically chased him around the stage to get it, but Kennedy offered only a lame handshake. Balz, Thursday morning, quoted from memory a line from the great, late Mary McGrory: “At the end of the convention, Jimmy Carter looked like an airline pilot whose passengers had all run off with the hijacker.”

Over at Politico, Jeff Greenfield recently recalled the notorious balloon-drop failure that same night:

“… the obligatory balloon drop became snarled in the rigging of Madison Square Garden; instead of the anticipated blizzard of red, white and blue, there came a pathetic dribble of occasional balloons, as though the convention hall had become afflicted with an enlarged prostate.”

One could make the plausible case that 15,000 journalists aren’t necessary to cover these conventions. But journalists aren’t simply here to cover a story. This is a convention for journalism, too — or, as Jonathan Alter put it when I ran into him Wednesday, they’re a “trade show.”

“Every big industry has a trade show. This is ours,” he said.

He’s been to 18 of these.

A few hours before Cruz took the stage, veteran yakker Chris Matthews strode into Quicken Loans Arena, ready for something like eight hours of punditry on MSNBC. I asked how many of these things — conventions — he’s been to. For a few minutes, he tried to do the calculation. Possibly 20. His first was when he was just a kid and worked as a busboy at the 1964 Democratic convention in Atlantic City.

“I got to hear Humphrey and Scoop and Gene McCarthy. It was a kick,” he said.

He got a little-boy’s mischievous smile on his face, and said, “I love all this stuff.”

There’s nothing quite like being in the arena when it’s throbbing with energy. Viewers watching on television may not have been able to sense how the tension built during Cruz’s speech. It was as if the hall was slowly realizing what was happening: Cruz had not yet endorsed Trump because he was never going to endorse Trump and had no interest in party unity and making nice and forgiving and forgetting.

First there was some heckling, some jeers, some chants of “Trump! Trump! Trump!” And then, as my sharp-eyed colleague Marc Fisher next to me pointed out, Trump himself came into the hall, stepping on the end of Cruz's speech. We were behind and to the right of Cruz, and could see his teleprompter, and he'd reached the last line of his speech. He paused. He seemed uncertain about what to do. People had jumped to their feet, as if witnessing a street fight. This wasn't heckling anymore; this was a full-throated roar of outrage. They shook their fists. Cruz said, "God bless America," and the arena became completely saturated in boos.

It was a Big Moment.