From Margin of Safety:
The research task does not end with the discovery of an apparent bargain. It is incumbent on investors to try to find out why the bargain has become available. If in 1990 you were looking for an ordinary, four-bedroom colonial home on a quarter acre in the Boston suburbs, you should have been prepared to pay at least $300,000. If you learned of one available for $150,000, your first reaction would not have been, “What a great bargain!” but, “What’s wrong with it?”.
The same healthy skepticism applies to the stock market. A bargain should be inspected and re-inspected for possible flaws. Irrational or indifferent selling alone may have made it cheap, but there may be more fundamental reasons for the depressed price. Perhaps there are contingent liabilities or pending litigation that you are unaware of. Maybe a competitor is preparing to introduce a superior product.
When the reason for the undervaluation can be clearly identified, it becomes an even better investment because the outcome is more predictable.