Although we don't believe in timing the market or panicking over daily movements, we do like to keep an eye on market changes -- just in case they're material to our investing thesis.

For those hoping for an early Santa Claus rally, today was a disappointment, as stocks generally posted modest losses as traders focused on preserving gains from what has been an extremely strong year. Yet while most of the major market indexes limited their losses to about half a percent or less on the day, Petroleo Brasileiro (NYSE:PBR), Cobalt International Energy (NYSE: CIE), and New Gold (NYSEMKT:NGD) all suffered much larger losses. Let's find out why those three stocks fell so dramatically today.

Petroleo Brasileiro dropped 11% as investors were dissatisfied with the Brazilian oil giant's new retail gasoline and diesel pricing policy. Shareholders had hoped that Petrobras would pull its fuel prices closer in line with international market prices, but despite a 4% rise in gasoline prices and an 8% jump in what it charges for diesel fuel, Petrobras didn't give any guidance on exactly how it would calculate future price adjustments. Obviously, with the company essentially acting as an arm of the Brazilian government in offering domestic subsidies to customers, it's difficult for investors to have any certainty about Petroleo Brasileiro's ability -- or even willingness -- to earn a profit.

Cobalt International Energy plunged 17% despite announcing that it had made a larger natural-gas discovery at its Lontra well off the shore of Angola that the company had initially estimated. You'd think that a small exploration and production company would see its shares soar on a bigger-than-expected find, but the problem is that given the difficulty in transporting natural gas, the value of the find relies on Angola building up sufficient local industry to use the fuel productively. Otherwise, Cobalt will have to rely on larger companies like Lontra partner BP (NYSE:BP) or peers like ConocoPhillips and Statoil that also have interests in Angola to look at building liquefied natural gas transportation facilities, an endeavor that would take years to pan out.

New Gold declined almost 10% as gold prices dropped close to their lowest levels of the year. New Gold has some of the lowest-cost gold production in the industry, with total cash costs of $280 per ounce in its most recent quarter and all-in sustaining costs of $779. But even with some margin of safety against prices in the $1,200 range, New Gold nevertheless will see profits hit if gold can't recover soon.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.