Despite a very strong first half, the stock market has continued to rise as investors apparently believe that the good times for corporate earnings and improving economic conditions can last at least a little while longer. Quietly, many market benchmarks have advanced to new highs, with the Russell 2000 small-cap index at an all-time record and the Nasdaq Composite hitting its best level in almost 13 years. As for the Dow Jones Industrials (DJINDICES:^DJI), its gain of more than 75 points put it less than 1% below its own all-time record close.

But amid the euphoria, it's important to keep a sense of perspective, and two of today's losing stocks make a more bearish case about prospects for the U.S. stock market. IBM (NYSE:IBM) was the worst performer in the Dow today, falling almost 2% after a Wall Street analyst downgraded the stock, citing sluggish emerging-market business activity. The drop comes after two straight days on which IBM has announced acquisitions, with today's purchase of private virtual-management company CSL following yesterday's closing on its buyout of SoftLayer Technologies, a cloud infrastructure company. Given the amount of influence IBM has on the Dow, a failure of the company to execute on its earnings-growth promises could point to difficulty throughout the tech sector, hurting one substantial contributor factor in the Dow's four-year bull market.

The other losing stock to take note of was Alcoa (NYSE:AA). Its 0.1% drop wasn't particularly noteworthy, except that yesterday afternoon, the stock traded 2% higher in after-hours trade immediately after releasing its quarterly earnings before giving back all of those gains and more by the end of the session. Even though one-time costs associated with facility closures and a settlement over allegations of corruption were what pulled the aluminum company's earnings into the red, Alcoa still faces the prospect of heavy supply overhangs that are hampering price pressure even as demand for aluminum from aerospace, commercial transportation, and other uses is on the rise. As long as overseas activity in construction remains relatively weak, it's hard to see the aluminum glut drying up.

Finally, beyond the Dow, Intuitive Surgical (NASDAQ:ISRG) was a huge decliner, falling 16% after issuing negative earnings guidance for the quarter. With an expected 27% drop in U.S. sales of its da Vinci robotic surgical systems, Intuitive is facing a crisis of confidence as would-be buyers respond to the FDA's investigation earlier this year and to tough questions about the effectiveness of the systems in performing procedures. When high-growth leaders start to falter, it's time to start thinking about whether the bull market is getting long in the tooth.

Fool contributor Dan Caplinger has no position in any stocks mentioned. You can follow him on Twitter: @DanCaplinger. The Motley Fool recommends Intuitive Surgical and owns shares of IBM and Intuitive Surgical. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.