Who Saw The Housing Bubble Coming? - Forbes.com: "There were, in fact, many warnings dating back more than seven years--but in the euphoria of rising home prices, no one listened. As time went by and no crash occurred, many of those doing the warning lost credibility or decided that perhaps they were wrong and moved on to other issues."
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For my part, I starting posting warnings from others about the housing bubble in August, 2004, but wasn't sure prices were about to collapse yet, and by summer of 2005 I was saying "did you hear a pop?"