The collapse of Fannie Mae, Freddie Mac and Lehman Brothers furnishes another opportunity to teach an investment lesson that still hasn’t been well learned and, for many people, never will be.

Diversify. Diversify. Diversify. And don’t put your 401(k) money in stock of the company you work for.

At mortgage giant Freddie Mac, about 8 percent of the money employees held in their 401(k) retirement plans was invested in Freddie stock at the end of 2006, Dow Jones Newswires reported this week.

Since then the shares have fallen from $34 to 45 cents. Millions of dollars in savings that people thought would be available for their retirement got wiped out. What’s worse, Freddie almost certainly will start to shrink and lay off some of the same people.

Managers at Fannie, another mortgage behemoth, wisely decided not to offer a company-stock elective for employees’ 401(k). But Fannie also had an employee stock ownership plan, which had $100 million in assets, mostly in Fannie shares, Dow Jones said. Those shares have followed Freddie into the toilet.

Lehman Brothers employees who loaded up on that company’s shares have also gotten plastered, and many will soon find themselves unemployed.

You’re already dependent on your employer for income. Don’t double the risk by putting your savings in the same basket.