Losing ground for the first time in five weeks, the S&P 500 Index (^GSPC 0.87%) lost less than 1 point, or less than 0.1%, to close at 1,649 as stocks head into the long weekend. Though posting slight losses, U.S. equities fared far better than most international markets, many of which fell following weak industrial data from China earlier this week. Doing their best impression of weak foreign stocks, these three companies ended as the worst in the S&P Friday.

Five-day periods just don't get much bleaker than the one GameStop (GME -3.94%) experienced this week. Today's 10.8% decline brought weekly losses to 19.2%, as Microsoft's Xbox One release was the initial downward catalyst for the stock and the new console will likely make the majority of the used-game industry out of business. Today's slip comes after GameStop announced quarterly results. Sales and income both fell, foreshadowing what could be a depressing future for shareholders. 

The second-largest laggard of the day, Salesforce.com (CRM 1.27%), lost 5.3% after a weak quarter of its own. The quarter wasn't so much weak as expensive: Increasing costs brought margins down. The company lost nearly $70 million in the first quarter, even though core revenue ticked upwards. But with operating expenses rising nearly 30% and interest expense surging 87%, it's tough to post an impressive quarter.

Lastly, shares of materials company Allegheny Technologies (ATI -1.58%) slipped 3.1% for a third straight day of losses. The week's earlier data from China initiated the slump, as investors fear sluggish industrial production in the Far East will either spread to the West or limit demand in high-growth markets abroad. Credit Suisse also lowered its price target for the stock today, citing an increase in nickel supply.