I'm (and economists) aren't saying that there are never $10 bills on the sidewalk, just that they don't stay there for long after someone notices them. Stock prices went down on the weekends for decades before someone noticed. But once it was noticed it stopped pretty quickly.

Understood. No offense, just being goofy.

I'm more interested in the persistent market inefficiencies; growth vs. value, January effect, low P/E, all interesting examples that haven't been arbitraged away.

Check out the behavioral finance literature, like the Shleifer book: http://www.amazon.com/Inefficient-Markets-Introduction-Behav...

I am just trying to double check to make sure I understood so bear with me. Are you telling me that stock prices systematically went down for decades and no one took this into account to make money? there is hope after all...

There's always someone on the other side of a trade.

Most of the time, the economist is right about that: the vast majority of ten-dollar-bill-appearing things found on sidewalks are obviously not real ten-dollar bills once you pick them up. They're advertisements for nightclubs or get-rich-quick schemes and whatnot.