The most interesting example is Royal Dutch vs. Shell Transport. Both companies had equal shares in Shell Oil - Shell Transport was in the S&P 500, Royal Dutch

The most interesting example is Royal Dutch vs. Shell Transport. Both companies had equal shares in Shell Oil - Shell Transport was in the S&P 500, Royal Dutch was not, and sold for much less, despite having identical economic interests - until they finally merged in 2005. When Yahoo was added to the S&P 500, its price jumped 24%. The name for this anomaly is usually called the "index effect", and whether it affects valuation is controversial.