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The witches in Shakespeare's Macbeth might have been chanting “bubble, bubble toil and trouble,” because that would be an appropriate chant on Wall Street this week. A few voices are emerging from those of the cheering section to question the longevity of this bull market.

Of course, those questions have been asked on and off all year long. Stock valuations have risen with stock prices, yet the long-expected correction has not materialized. Many of the most bearish traders have thrown in the towel in recent weeks and become bulls.

And a lot of people see that as something to worry about.

There's an expression among traders that stocks tend to “climb a wall of worry.” That means prices go up when a significant number of players express worries the market is over valued and a correction is due.

Kind of like the situation several months ago, when there were plenty of bears warning of a correction. But when everyone starts singing “Happy Days Are Here Again,” look out.

Those who were in the market in early 2000, at the tail end of the dot-com boom, remember the refrain “this time it's different,” used to justify buying stock in companies with no earnings but a dot-com at the end of their name. A few months later it was clear that nothing was different.

Fed pumping

Since the 2008 financial crisis, the Federal Reserve has been buying bonds and holding down interest rates. Some critics point out that has inflated stock prices, since companies can cheaply borrow money to buy their own stock, pumping up the price.

With bonds and CDs paying next to nothing, more money has flowed into the stock market, pushing prices ever higher. As long as the Fed keeps interest rates at zero, the argument goes, stocks are destined to go higher.

But this month, there have been a few people on Wall Street waving the caution flag.

“With the sentiment number so outrageously high and with the margin debt so off the charts, we really feel this is a very risky time,” Brad Lamensdorf, manager of the Range Equity Bear Fund, told Yahoo Finance this week.

Lamensdorf said he thinks a 10% to 20% correction is in order to get the market back to where it should be. In early September billionaire investor Sam Zell told CNBC that he believes a market pullback is on the horizon.

"People have no place else to put their money, and the stock market is getting more than its share,” he told the network. “It's very likely that something has to give here."

Avoid panic

This week there were signs that a correction of some type may have already begun. The stocks of small companies have suffered a sharp decline.

The Russell 2000, a stock index made of up small companies, suffered a “death cross,” a chart event that in the past has signaled the onset of a bear market.

But if there is a sharp correction, people with retirement accounts full of mutual funds shouldn't panic. After all, unless you have an extremely short retirement time line, you're in it for the long haul.

Long-time market bull Jeremy Siegel says the market may face some rough sledding in the weeks ahead, but should continue to move higher over time. Even investors who rode out the catastrophic market decline of early 2009 were well ahead of the game by 2010.

The point is to not be shocked if the market gives up some of what it has gained this year. What should consumers with retirement accounts packed with mutual funds be doing?

It might be wise to review your retirement portfolio and discuss strategy with your financial advisor. If you are sharply higher in some of your positions, it might be prudent to take some profits. 

Remember -- buy low, sell high. Taking some profits could give you the cash you need to wade back into stocks after they've come back down to earth.

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