Suppose you have someone that says "amazon is overvalued, there's no way they are worth more than $400B" over and over again for years as Amazon goes from $600B to $900B. I'd still keep an open mind as long as the market in general was irrationally exuberant over that period. It's only once the market as whole returned to sanity and most stocks went down, but amazon continued to raise that I'd say that guy probably doesn't know what he's talking about.