Consider KSW Inc. (KSW), a stock we have previously discussed as a potential value play. The company's CEO announced a plan to exercise 105,000 of his options, and then sell those shares in the market. The announcement was made several months ago at a time when he did not have inside information. Furthermore, the rest of his 750,000 shares will remain under his ownership, continuing to leave him a stake of over 10% in the entire company.
The plan was announced in March, and resulted in almost no change in the stock price. As the CEO has been selling the announced shares, however, the stock price has been dropping. The company only trades a few thousand shares per day; therefore, selling 100K+ shares over three or four months can result in significant price movements. In this case, shares have fallen some 20% since the share sales began. Now that the sales are practically complete, the share price can breathe again, as the supply and demand of the shares of KSW are free once again to find a more normal equilibrium.
Many financial experts regard stock prices as fully reflecting the values of the underlying businesses. This assertion must be thrown in doubt, however, in cases such as this one, where shareholders controlling large blocks of shares are dumping their stock over very short periods of time. The market is not efficient here; excess supply of shares is driving down the price, offering opportunities for those who don't believe the market is efficient.
Disclosure: Author has a long position in shares of KSW