After peaking near $150 a barrel over the summer, oil prices have tumbled nearly 60%. As anyone who has pulled up to a pump can attest, gasoline prices have abruptly returned to earth as well. While the declines have been a boon to consumers, not everyone is celebrating; namely, mutual fund managers who loaded up on energy stocks.

The average U.S. stock fund specializing in natural resources is down 50% this year, making it the second-worst group among all U.S. stock funds after communications funds, according to Morningstar. Unless you're adept at market timing, it's a sore reminder of the risks involved in buying sector-specific funds.

But even investors who stayed away from specialty funds might have gotten dinged. Several diversified stock funds also bet heavily on oil, with upward of 30% of their portfolios in energy. Morningstar recently rounded up a list of funds with some of the biggest energy bets, and lo and behold, many of the funds that were among the best performers among peers in the first half of this year now rank at the bottom.

This includes CGM Focus (CGMFX), which has produced double-digit returns every year since 2001 (with the exception of 2002). This year, it's down 48%, trailing peers and the S&P 500. As of June 30, manager Ken Heebner had about 58% of the fund's assets invested in energy. Top holdings included Peabody Energy (BTU) and Brazil's Petrobas (PBR). Back in June, Morningstar analyst Wenli Tan noted that Heebner "thinks consumption will continue to grow in emerging markets and has positioned the portfolio accordingly."

More recent portfolio data for CGM Focus aren't publicly available yet, and we couldn't reach Heebner to discuss his current holdings. But the Boston Globe recently reported that Heebner sold off most of his big stakes in energy, metals and other commodities over the summer as these holdings lost ground. "[Heebner] says those positions, which had dominated the CGM Focus portfolio, are all but gone now," according to the Globe.

"It's hard to avoid the conclusion that these funds' big energy stakes are a major factor behind this whiplash-inducing reversal in performance," Morningstar analyst David Kathman said in a recent report. "The funds with the biggest swings also tend to have a lot of holdings in other commodities."

Another well-regarded fund in the group is FPA Capital (FPPTX), which had 44% of its assets in energy as of Sept. 30. To date, it's down 32%. To manager Bob Rodriguez's credit, though, he's beating his peers in the midcap value category as well as the S&P 500, which is down 39% through Friday's close.

The lesson? Even professional money managers have trouble getting out of minefields in time to avoid trouble, which is why having a concentrated fund can be risky on a short-term basis.

This doesn't mean you should jump ship, although many investors are pulling money out of mutual funds. Longer term, CGM Focus has a great record: Its 10-year annualized return through Sept. 30 is 26%, well ahead of the rest of the market. FPA Capital also has an impressive long-term record, with a nearly 12% annualized return for the past decade. So keep in mind that taking a loss because of a fund's short-term missteps can be a risk in itself.