Interesting that Buffett wrote this in 1958 (in letter to investors of his hedge fund before he took over Berkshire Hathaway). Buffett still going strong while others blew themselves up through just about every market cycle over the past 5+ decades. 

During the past year; almost any reason has been seized upon to justify “Investing” in the market. There are undoubtedly more mercurially-tempered people in the stock market now than for a good many years and the duration of their stay will be limited to how long they think profits can be made quickly and effortlessly. While it is impossible to determine how long they will continue to add numbers to their ranks and thereby stimulate rising prices, I believe it is valid to say that the longer their visit, the greater the reaction from it.

I make no attempt to forecast the general market - my efforts are devoted to finding undervalued securities. However, I do believe that widespread public belief in the inevitability of profits from investment in stocks will lead to eventual trouble. 

Buffett wrote this the following year, in 1959:

Perhaps other standards of valuation are evolving which will permanently replace the old standards. I don’t think so. I may very well be wrong; however, I would rather sustain the penalties resulting from-over-conservatism than face the consequences of error, perhaps with permanent capital loss, resulting from the adoption. of a “New Era” philosophy where trees really do grow to the sky.