5 portfolio time bombs

Remember when hundreds of WorldCom and Enron employees lost their entire savings? It was a brutal reminder of how dangerous it is to bank your retirement on a single company, but the lesson seems to have been forgotten.

According to a recent study by Fidelity, employees who own shares of their company dedicate 29 percent of their 401(k) to it on average. "People keep company stock because they know the firm and are invested in it in other ways," says Stephen Horan, head of private wealth for the CFA Institute.

It's also easy to overload on your employer's shares without meaning to, particularly if your 401(k) match is in company stock or you get stock options.

Plus, if you work for a major corporation, you likely own even more of your company stock through any large-cap funds you own.