John Hussman is a brilliant economist. His commentaries are insightful and educational. Although he's been cautious on the markets for a while, he may have just given a prescient and timely warning for all investors. Below is just an excerpt with a link to his full commentary.
The Music Fades Out
The problem with financial bubbles is not that they are difficult to recognize, but that they can only emerge if the majority of market participants are willing to excuse or dismiss the extremes.
Intuitively, if overvaluation alone was always immediately followed by market losses, it would be impossible for the market to reach valuation extremes like 1929, 2000 and today, because the market advance would have been halted by much lesser overvaluation.
Presently, the dispersion we observe in market internals suggests that investors are becoming increasingly selective, and that their psychology has subtly shifted away from speculation and toward risk-aversion.
The music is fading out, and a trap-door has opened up in the floor, but they're still dancing.