"Being wrong on your own, as Keynes describes so eloquently in Chapter 12 of The General Theory, is the cardinal crime for an investment manager. To avoid this, the professionals try very hard to ensure that if they are going to run off any cliff they will: a) have a lot of company; and b) that most of the company will be one step ahead."
Both of these are from Jeremy Grantham's November letter to clients, available here as a PDF. An excellent letter from Ben Inker, a younger colleague of Grantham, is part of the same document. Both are well worth reading, as usual.