On: Today the Dow dropped 1000 points in about ten minutes.

[jey]

As I understand it, all it takes is one share to have been sold at that price. Probably indicates someone having a "sell n shares at any price" order where n > buy orders at that moment..?

Regardless of the details, this is an example of how markets aren't perfectly efficient.

[eru → jey]

> Regardless of the details, this is an example of how markets aren't perfectly efficient.

No. Just that the last reported transaction may not be `true' price.

There are other arguments against efficiency of markets, but this isn't.

[yan → jey]

I don't think anyone claims the markets are 'perfectly' efficient. Even the people that believe they are efficient and choose to participate argue that they are exploiting the inefficiencies.

[orblivion → yan]

Furthermore the real question is, could the market possibly be made more efficient if some "expert" were there deciding when a trade was a bad idea?

[eru → orblivion]

Shouldn't they be experts on both sides of all (or at least most) trades already?

[WingForward → orblivion]

Probably.

[jey → WingForward]

  [citation needed]

"Experts" are overrated. The whole notion is fundamentally backward-looking.

You might as well consult a historian.