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Stocks have been trading at or near record highs -- but not everyone is so certain that the acceleration will continue.

One measure of optimism, fund flows, showed a reversal last month, with fund investors -- a group that largely consists of middle-class American retirement savers -- pulling more than $2 billion net in May from domestic stock mutual funds and ETFs.

Many professional investors are expressing concern as well. The following five investors, all of whom saw signs of the 2008 financial collapse, are now forecasting declines ahead, many in quite dire terms.

While none should send you screaming for the exits -- after all, many experts were making similarly grim predictions a year ago -- their jitters should serve as a good reminder to revisit your portfolio and make any needed adjustments to your investing mix, particularly if you're likely to need the money soon.

When exactly will the good times end? That's a futile question, says Rob Arnott, founder of Research Affiliates, an investment strategy firm.

“People always ask: What is the catalyst to cause the market to break? Catalysts are, by definition, a surprise,” he says. “Even after, you can’t always define it.”

"It’s going to be agonizing.”

Tom Forester, chief investment officer at Forester Capital Management

courtesy of Forester Capital Management

Tom Forester has already predicted one stock downturn: Before the market collapsed in 2008, Forester sold off enough bank stocks to bring his fund's financial holdings down to 5%—at a time when the financial sector made up 20% of the S&P 500 index. Institutional Investor highlighted his fund as “the sole long-only mutual fund in the U.S. to gain in 2008.”

Now Forester isn’t sleeping too well again.

The last two crashes, he says, were sparked by one industry’s failure—tech in 2000, and housing in 2008. This go-round, he fears almost every sector has become overvalued. In fact, nine out the S&P’s 10 sectors are more expensive than their historical 10-year average, according to a recent FactSet report. (Telecom is the only exception.)

“The next time we see a bear market, it’s going to be agonizing,” Forester says. “There won’t be anywhere to hide on the way down.”

"It’s going to collapse."

Jim Rogers, investor

Sergei Savostyanov—Getty Images

Months before the 2008 stock market collapse, Jim Rogers—the legendary investor who founded Quantum Fund with George Soros—was shorting the shares of Wall Street investment banks. Now he says a crash is again imminent.

“Some stocks in America are turning into a bubble, the bubble is going to come, and then it’s going to collapse,” he told Business Insider this month on its weekly show “The Bottom Line.”

“You should be very worried,” Rogers said.

The 2008 financial collapse was as bad as it was because of the debt at the time. But the debt borne today - in the U.S. and China and even the Federal Reserve - dwarfs that of 2008, he said.

With the debt so high, Congress could have less room to cut taxes or to borrow and spend to help re-start growth in the midst of a recession. In the long run, high levels of government debt also typically translate into higher interest rates, making it more difficult for both government and business to invest in the economy.

“How big a crash could we be looking at?” Business Insider CEO Henry Blodget asked.

“It’s going to be the biggest in my lifetime,” the 74-year-old investor said.

"Asset holders will lose 50%."

Marc Faber, Swiss investor and author of the monthly newsletter, “The Gloom, Boom and Doom Report”

ISTANBUL, TURKEY - OCTOBER 15: Swiss investor Marc Faber speaks during an exclusive interview after he attended capital market conference in Istanbul, Turkey on October 15, 2015. (Photo by Sebnem Coskun/Anadolu Agency/Getty Images)
Anadolu Agency—Getty Images

Full disclosure: Marc Faber is always preparing for a stock apocalypse. (That's why he’s commonly referred to as “Mr. Doom.”) Still, he insists, there’s method to his misery. And right now he sees two red flags flapping in the market.

One: On the New York Stock Exchange, there are currently more stocks purchased on margin—that is, with investors borrowing money to buy—than since at least the 1950s. That tends to happen when the stock market is expensive, as it is today.

Prices are actually out of control, Faber says. The historical average price-to-earnings ratio is around 17—but it's around 30 today.

Once people start selling, Faber warns grimly, there will be an avalanche. “I think a realistic scenario is that asset holders will lose 50% of their assets," Faber says. "Some people will lose everything.”

His other major concern is that only a small number of stocks are driving the bulk of the stock market’s ascent. Indeed, just five companies accounted for almost a third of the S&P 500’s total gains in 2016. This means that investors are relying on fewer companies to carry the market, he points out.

“If only a handful of shares are moving up, it’s a sign,” Faber says. “The market isn’t healthy.”

“All markets are increasingly at risk.”

Bill Gross, manager of Janus Henderson Global Unconstrained Bond Fund

Patrick T. Fallon—Bloomberg via Getty Images

“Don’t be mesmerized by the blue skies,” Bill Gross, the well-known bond investor, said in his June investment outlook letter. “All markets are increasingly at risk.”

Before the storm of 2008, Gross stashed away $50 billion in cash for when his trading partners wanted payment from his firm. Now he worries that the risks have piled up again.

Investors have resorted to "making money with money," Gross says, with more money funneled into the financial economy than the real economy. When you match that with the U.S.'s high debt, aging population, and the automation of labor, you have a big problem for productivity, Gross argues. And productivity, he says, is the long-term driver of economic growth and profits.

The current set of challenges, Gross says, "promise to stunt U.S. and global growth far below historical norms."

"There's not enough fear."

Rob Arnott, founder of Research Affiliates, an investment strategy firm

Tim Boyle—Bloomberg via Getty Images

Rob Arnott is dubbed “the godfather of smart beta," the popular passive investing strategy that pulled in more than $55 billion last year. Arnott gives investment advice to people all over the planet. In a 2007 interview with Bloomberg, he said a recession was coming.

Lately he’s not feeling too good about the U.S. stock market. Investors are devoid of necessary caution, he says. “In the United States, there’s not enough fear,” Arnott says. “There’s a sense that the risks are not there.”

Yet a shift could come at any moment, he adds. “Markets move with the changes in perception,” Arnott says. “One bad thing could cause a downturn.”

And we may be due for that one bad thing, he cautions. "It’s hard to paint a rosy picture for any long-term investor. The market is just too expensive,” Arnott says. “At any point it might roll over and die.”