Apple Inc closed at an all time high of 702.10 on September 19. At the time, $AAPL was up 73% for 2012 and accounted for nearly 5% of the SP500 Index.
On the same date, the $SPX close at 1461 up 16% for the year and $AAPL accounted for approximately 20% of the gain.
That is to my recollection an unprecedented single stock skew for an index made up of 500 of the largest US public companies (Does anyone recall $XOM weighting at the late ’07 early ’08 highs?).
Since September 19th, $AAPL has fallen 24%.
Meanwhile, the $SPX has fallen only 2.8% and $AAPL has accounted for around 35% of the that move lower.