Not a single sector in the stock market advanced on Tuesday, as Wall Street considered continuing tumult in Ukraine and signs of slowing growth in China's breakneck economy. Unfortunately, the complexity of modern markets makes geopolitics and international trade just two of the many factors everyday investors should consider as they build their portfolios. But even supremely balanced portfolios can't diversify away all risk, and sometimes you're left with the short end of the stick; this might sound familiar for people owning Juniper Networks (NYSE:JNPR), Pioneer Natural Resources (NYSE:PXD), and Johnson Controls (NYSE:JCI), three of the worst performers in the S&P 500 Index (SNPINDEX:^GSPC) today. The S&P itself was down 9 points, or 0.5%, to end at 1,867 Tuesday.

Juniper Networks, a $12 billion network communications company, saw its shares shed 3.4% today as activist hedge fund Elliott Management disclosed that it had added on to its position in the company. Elliott has already goaded Juniper into a dividend and a share repurchase program since building its original 6.2% stake earlier this year, and Juniper's board of directors is probably getting migraines at the thought of any additional pressure. While activist investors are often idolized and obsessed over in the financial media, individual investors should remember that activists don't care about the long-run and can mislead management with their myopic vision and fleeting changes. 

Shares of oil and natural gas producer Pioneer Natural Resources slumped 3.3% today as the price of natural gas fell by about 1%. Pioneer Natural Resources is one of those companies that isn't yet profitable, an uncomfortable fact that can be confusing considering the stock was the top performer in the entire S&P 500 last year. You can say a lot about Wall Street -- it tends to be awful greedy and shortsighted -- but it isn't dumb, and neither is Pioneer Natural Resources, which could double its output by 2018 by tapping newfound shale reserves in Texas. 

Finally, stock in auto parts maker Johnson Controls dropped 2.8% Tuesday. Like Pioneer Natural Resources, the stock had a remarkable 2013, and anyone who believes in "reversion to the mean" will tell you that makes market-beating growth in 2014 harder to achieve. Because the stock market is as much about expectations as it is results, when companies beat results day in and day out, expectations become higher. Eventually companies either run out of aces or room in their sleeves, and stocks "revert" to performing in line with their peers or the rest of the market. With rival Illinois Tool Works expecting just 2% to 4% sales growth this year, Johnson Controls shareholders are taming their own expectations in response.

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.