One thing the past few months have taught us is this: No company is bulletproof. 

In March, shares of Bear Stearns pancaked from $70 to $10 in three weeks after the government engineered a bailout of the investment bank. Concerns that the government will also bail out mortgage giants Freddie Mac and Fannie Mae have caused their share prices to sink more than 70% in the past three months.

These reversals of fortune offer a cautionary lesson for employees who are allowed to invest in their company's stock through their 401(k) plan.

"No company, no matter how large, how well thought of or how well-capitalized, is immune from bad things happening," says Christopher Jones, chief investment officer for Financial Engines, which provides advice to 401(k) plan participants. If your 401(k) is loaded up with company stock and your employer falls on hard times, he says, "You could easily find yourself not only bereft of retirement savings but also your job."

Enron's spectacular collapse in the fall of 2001 hammered home the risks of owning too much company stock. Some Enron employees who were heavily invested in company stock saw their retirement savings evaporate overnight.

But seven years after the Enron implosion, thousands of employees still own unhealthy amounts of their companies' stock. Thirty-six percent of participants in 401(k) plans that offer company stock as an investment option have more than 20% of their savings in company shares, according to a Financial Engines analysis. Older workers were even more likely to own an excessive amount of company stock, the analysis found. A quarter of 401(k) plan participants over 60 had at least half their 401(k) portfolios invested in company stock. 

In general, workers shouldn't have more than 10% of their savings in any single company stock, including their employer's, says Pamela Hess, director of retirement research for Hewitt Associates. 

Fannie Mae and Freddie Mac don't offer company stock as an investment option in their 401(k) plans, according to spokesmen for the companies. But nearly half of large employers still include company stock in their 401(k) plans, according to a 2007 survey by Hewitt Associates. Of those, 82% place no restrictions on the percentage of contributions that employees can invest in company shares.

Even workers who don't select company stock as an investment option could end up with an unbalanced portfolio if their employers make matching contributions in the form of company stock. 

About 11% of companies match 401(k) contributions with company shares, according to Hewitt Associates. If you receive company shares as a match, you should periodically sell some of those shares and reinvest the money in other funds, Jones says.

Investing in a target-date fund won't relieve you of this task. Target-date funds, also known as life-cycle funds, invest in a mix of stocks, bonds and money funds, based on when you plan to retire. They're popular with workers who want to put their investment plans on auto-pilot. But if your contributions are matched with company stock, putting your plan on cruise control could get you into trouble. 

With a typical company match of 50 cents on the dollar, Jones says, "You could end up with a portfolio that's a third company stock and two-thirds life-cycle fund."

To prevent this, workers who receive their match in the form of company stock should go into their 401(k) plans a couple of times a year, sell company shares and reinvest the proceeds in the target-date fund, Jones says. If your company stock is doing extremely well — which means it will eventually make up a larger percentage of your portfolio — you might need to do this more often.

In the past, many companies placed restrictions on when employees could sell their matching shares. Some workers had to wait until they were 50 or older, or could sell their shares only a couple of times a year. After Enron, though, most employers lifted such constraints. Two-thirds of employers let workers sell matching shares at any time, up from 46% in 2005.

Yet, most workers rarely touch their 401(k), Hess says. Last year, only 19% of participants made any changes in their portfolios, Hess says. But if your employer matches your contributions with company stock, she says, "You have to be a more active user of your 401(k) than the average investor."