“We've long felt that the only value of stock forecasters is to make fortune tellers look good. Even now, Charlie and I continue to believe that short-term market forecasts are poison and should be kept locked up in a safe place, away from children and also from grown-ups who behave in the market like children.”

2 November 1959
Letters to Shareholders (1957 - 2012)
Context: 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 investments in stocks will lead to eventual trouble. Should this occur, prices, but not intrinsic values in my opinion, of even undervalued securities can be expected to be substantially affected.

Though often attributed to Galbraith, as early as 1988 in U.S. News & World Report, the earliest publications of this statement, in The Bulletin (1984) and Reader's Digest (1985) attributes it to Ezra Solomon.
Misattributed

2010s, 2016, September, First presidential debate (September 26, 2016)
Context: We have the worst revival of an economy since the Great Depression. And believe me: We're in a bubble right now. And the only thing that looks good is the stock market, but if you raise interest rates even a little bit, that's going to come crashing down.

The Silver Chair (1953), Ch. 16: The Healing of Harms
The Chronicles of Narnia (1950–1956)

Source: Culture and Value (1980), p. 17e

Issing commenting the situation in Eurozone in 2016 http://www.marketwatch.com/story/architect-of-the-euro-now-says-it-is-a-house-of-cards-2016-10-19

2016, September 2016, First presidential debate (September 26, 2016)

Source: The Theory of Gambling and Statistical Logic (Revised Edition) 1977, Chapter Nine, Weighted Statistical Logic And Statistical Games, p. 295

Source: Debunking Economics - The Naked Emperor Of The Social Sciences (2001), Chapter 10, The Price Is Not Right, p. 216