If it ain’t broke, don’t fix it.

The time-tested adage appears to be the lesson from Coca-Cola’s disastrous introduction of “New Coke” 30 years ago. Except that in 1985 Coca-Cola indeed thought that its signature brand was broken.

Although Coca-Cola remained the world’s best-selling soft drink, rival Pepsi-Cola continued to gain market share in the 1970s and early 1980s, thanks in part to its aggressive “Pepsi Challenge” campaign in which consumers taking blind taste-tests were surprised to learn they preferred the flavor of Pepsi. To the shock of Coca-Cola, internal taste-tests yielded the same results. Company executives grew convinced that its soda’s taste—not its rival’s advertisements targeting the “Pepsi Generation”—was the reason for its declining market share.

Since its introduction in 1886, Coca-Cola’s secret recipe had been tweaked several times—such as when changing sweeteners from cane sugar to beet sugar to corn syrup—but its taste had remained constant. While the company was developing the unique formula for Diet Coke, which was introduced in 1982, it found in top-secret taste-tests that a sweeter version of the concoction beat not only Pepsi, but the classic version of Coke. Executives decided to make a risky change.

On April 23, 1985, Coca-Cola Company chairman and CEO Roberto Goizueta stepped before the press gathered at New York City’s Lincoln Center to introduce the new formula, which he declared to be “smoother, rounder, yet bolder—a more harmonious flavor.” The press, however, said what Goizueta couldn’t admit: New Coke tasted sweeter and more like Pepsi.

Had it been an opera, the Lincoln Center performance would have been a tragedy to devoted fans of Coke’s original formula. Rather than divide its market share between two sugar sodas, Coca-Cola discontinued its 99-year classic recipe and locked Formula 7x away in an Atlanta bank vault with the intention that it never again see the light of day.

“Some may choose to call this the boldest single marketing move in the history of the packaged goods business,” Goizueta said. “We simply call it the surest move ever made.” Coca-Cola president Donald Keough echoed the certainty: “I’ve never been as confident about a decision as I am about the one we’re announcing today.”

While Goizueta and Keough toasted each other with cans of New Coke, the news was already beginning to fall flat. On the New York Stock Exchange, shares of Coca-Cola dropped while those of its rival rose. Pepsi gave its employees the day off and declared victory in full-page newspaper advertisements that boasted, ‘‘After 87 years of going at it eyeball to eyeball, the other guy just blinked.’’

New Coke left a bitter taste in the mouths of the company’s loyal customers. Within weeks of the announcement, the company was fielding 5,000 angry phone calls a day. By June, that number grew to 8,000 calls a day, a volume that forced the company to hire extra operators. “I don’t think I’d be more upset if you were to burn the flag in our front yard,” one disgruntled drinker wrote to company headquarters. At protests staged by grassroots groups such as “Old Cola Drinkers of America,” consumers poured the contents of New Coke bottles into sewer drains. One Seattle consumer even filed suit against the company to force it to provide the old drink.

The outrage caught Coca-Cola executives by surprise. They had hardly made a rash decision unsupported by data. After all, they had performed 190,000 blind taste-tests on American and Canadian consumers. The problem, though, is that the company had underestimated loyal drinkers’ emotional attachments to the brand. Never did its market research testers ask subjects how they would feel if the new formula replaced the old one.

Seventy-nine days after their initial announcement, Coca-Cola executives once again held a press conference on July 11, 1985—this time to announce a mea culpa and the return of the original formula, which hardly had time to gather dust in its Atlanta bank vault, under the label “Coca-Cola Classic.” “Our boss is the consumer,” Keough said. “We want them to know we’re really sorry.” The news was so momentous that television networks broke into normal programming with special reports.

Coca-Cola Classic quickly outsold New Coke and within a few months had returned to its position as the top-selling sugar cola, ahead of Pepsi. The company rebranded the new formula “Coke II” in 1990 before it was eventually abandoned in 2002. In spite of the blowback, Coca-Cola emerged from the fiasco with its market position actually strengthened as consumers rediscovered their attachment to the iconic brand.

“The simple fact is that all the time and money and skill poured into consumer research on the new Coca-Cola could not measure or reveal the deep and abiding emotional attachment to original Coca-Cola felt by so many people,” Keough admitted. The blunder was so colossal that some thought it must have been an intentional marketing gimmick. “Some cynics say that we planned the whole thing,” Keough said. “The truth is we’re not that dumb and we’re not that smart.”