This is the Reason the S&P 500 Headed Lower Today

It's not often that you can blame Apple (NAS: AAPL) for the S&P 500's (INDEX: ^GSPC) woes -- Apple is up about 70% year-to-date -- but today's drop of 3.26 points (0.22%) to 1456.89 is primarily Apple's fault. The tech bellwether reported today that it completely sold out of its inventory of 5 million iPhone 5s over the weekend. However, Wall Street was looking for much more -- closer to 8 million. It should be noted that these figures don't include pre-ordered phones, but it raises concerns about the ability of Apple's suppliers to boost capacity to meet demand.

Besides Apple, let's have a look at a few other large movers within the S&P 500.

Paper manufacturers, like International Paper (NYS: IP) , had a fantastic session, bucking the S&P 500's downdraft, following multiple price target upgrades within the sector from Credit Suisse. The brokerage firm noted that International Paper and many of its peers were able to pass along $50/ton price increases in September, which should help curtail higher input costs. I'm not sold on the paper sector just yet, but this does address a major concern that I had regarding rising input costs.

Apple's pain also turned into Google's (NAS: GOOG) gain. Keep in mind that, while the Apple iPhone is the best-selling single device in the world, Google's Android operating system is still the most prominent operating system by a mile! Within the U.S., Google's OS market share dominates Apple, 52.2% to 33.4% according to comScore. Today's "disappointment" from Apple that it "only" sold 5 million iPhones since Sept. 21 may bode well that Google's OS won't be losing market share to Apple's iOS6 anytime soon.

Virtualization software company Citrix Systems (NAS: CTXS) is having one of its rougher days within the S&P 500, down more than 5%, following a downgrade from Morgan Stanley to "equal-weight" from "overweight." Morgan Stanley cites weakness in the PC market, which could translate into a slowdown in Citrix's organic license and billing growth. On the other hand, my Foolish colleague Keith Speights feels Citrix is a well-oiled machine that could be well on its way to becoming a virtualization powerhouse.

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Fool contributorSean Williamshas no material interest in any companies mentioned in this article. You can follow him on CAPS under the screen nameTMFUltraLong, track every pick he makes under the screen nameTrackUltraLong, and check him out on Twitter, where he goes by the handle@TMFUltraLong.The Motley Fool owns shares of Apple and Google.Motley Fool newsletter serviceshave recommended buying shares of Apple and Google, as well as creating a bull call spread position in Apple. Try any of our Foolish newsletter servicesfree 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 adisclosure policy.

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