5 portfolio time bombs

Past performance has never been, and will never be, a reliable guide to how well a fund will do in the future. Yet investors can't resist big returns.

International stocks have trounced the S&P 500 by an average of nine percentage points a year since 2002. Sure enough, this past January investors poured $10 billion into foreign-stock mutual funds, vs. just $5 billion into U.S. stock funds.

"Investors tend to take notice of a fund once it's had a high return," says David Silva, editor of Morningstar.com. Trouble is, a hot streak might run for several years, but at some point you'll likely encounter a dramatic blowup, or at least far smaller gains once returns go back to normal (which you can count on).

And popularity has a price: The more money that flows into a fund, the harder it becomes for the manager to react quickly to market changes, making it even more likely that the returns will sag in the near future.