It's been a big help to the consumer—but maybe it should be a big worry to stock investors. That would be falling gas prices.

Over the past two months, unleaded gasoline prices have dropped by 18 percent, while the S&P 500 has risen nearly 4 percent. And here's why that could be trouble: Carter Worth, chief market technician at Oppenheimer, points out that every time the S&P has corrected by 5 percent or more since the market's March 2009 low, gasoline has either led that drop, or fallen with the market.