Once every decade or so investor credulity reaches a point where even seasoned money managers buy into the notion of "one decision" stocks -- that is, shares of companies so insanely great that they're virtually guaranteed to keep going up. Valuation is irrelevant, as is the state of the economy. Nothing matters but the unbeatable business model/technology/visionary leadership of such companies, so owning their stocks is as close to risk-free investing as it's possible to get.

Back in the 1970s it was the "Nifty 50" (mostly American) multinationals that were taking over the world. In the late 1980s junk bonds, believe it or not, assumed this role (Institutional Investor magazine actually ran a cover story titled "The Unsinkable Junk Bond"). In the late 1990s it was a handful of hot tech companies that made up for their lack of earnings with surging numbers of "eyeballs." In the 2000s it was five or six big banks whose traders were, as CNBC liked to put it, "the smartest guys in the world."

All of those delusions ended in tears. Yet here we are again, with a market supported by a few fast-growing tech companies that everyone now assumes to be bulletproof. Facebook, Amazon, Netflix and Google, -- known as the FANGs -- along with Apple, have largely sustained an otherwise thin and declining Dow, S&P 500 and NASDAQ over the past year.

But now the FANGs are developing some cavities. Amazon, for instance, nearly tripled in 2015 from an already rich base. So it's not a surprise to see it correct a bit in the new year. Still, 60 points is a pretty impressive drop.

One catalyst for the trend change is analysts beginning to tell clients to take some profits:

Amazon.Com, Inc. (AMZN) Shares Plunge On Downgrade And General Market Drop

(IR.net) - After shares of Amazon.com, Inc. (NASDAQ:AMZN) hit new 52-week highs to close off 2015, 2016 seems to be getting off to a rocky start for the company's stock, along with the market in general. Shares of Amazon.com are down $14.89 or 2.2% following a downgrade by Monness Crespi & Hardt from a 'buy' rating to a 'neutral' rating.

From a total of 26 analysts covering Amazon.com stock, 23 rate it a "Buy", 0 a "Sell", and 3 a "Hold". This means that 88% of the ratings are positive.

There are at least two red flags here. One is that despite the overwhelmingly positive analyst consensus, a downgrade from a single boutique research shop can knock $20 billion from its market value. The second red flag is analyst opinion generally. Market historians should commission a study to see if any stock has ever risen when 88% of analysts rated it a buy and no one rated it a sell.

Meanwhile Apple, after passing Exxon and GE to become the most valuable stock ever, has been trending down for several months and lately has hit an air pocket on stalling iPhone sales.

Apple Falls on Nikkei Report That Sees iPhone Production Cut 30%

(Bloomberg) - Apple Inc. fell after Japan's Nikkei Asian Review reported the U.S. company would reduce the output of its latest iPhones by about 30 percent in the first quarter of 2016.

Shares of the world's most valuable company dropped 2.8 percent to $102.42 at 12:17 p.m. in New York. They declined 4.6 percent in 2016.

Apple had initially told parts suppliers to keep production of the iPhone 6s and 6s Plus models for the January-March period at the same level as for their predecessors iPhone 6 and 6 Plus a year earlier, the Nikkei reported. But inventories of the new models, which debuted in September, have piled up at retailers in developed markets like China and Europe amid lackluster sales while an increase in the dollar against emerging markets currencies has made the phone more expensive in those countries, the magazine said.

"This is an eye-opening production cut which speaks to the softer demand that Apple has seen with the 6s out of the gates," said Daniel Ives, managing director at FBR Capital Markets. "The Street was bracing for a cut, but the magnitude here is a bit more worrisome and speaks to a soft March quarter."

In past "one-decision stock" eras, the end of those companies' dominance either coincided with or flat-out caused a general market melt-down. Let's see how it goes when the remaining FANGs turn out to be mortal.