For many investors, the best bet has been to put their money in an index fund that tracks the S&P 500 and just let it ride. However, buying and holding an S&P 500 index fund is very different from buying and holding the 500 companies that make up the index. For many reasons.

One key reason is that the S&P 500 experiences quite a bit of turnover. Names are added and deleted regularly.

"To be clear, not every S&P 500 deletion was the result of a 'problem stock'," wrote JP Morgan Asset Management's Michael Cembalest in September. "Actually, most deleted companies were not the result of a problem, and reflect benign index removals because: they were acquired at a premium to their current price; they merged with other companies in the index; or, they reincorporated outside the US."

This is not to say there haven't been a lot of S&P 500 distressed business "problem stocks," which Cembalest characterized as "the S&P 500 deletions that were a consequence of stocks that failed outright, were removed due to substantial declines in their market value, or were acquired after suffering such a decline."