It could take a decade or more for Wal-Mart stock to recover from last week's $18 billion plunge. But that's not the most important lesson for investors here…

Wal-Mart stock lost $18 billion in value this Wednesday after management revealed the company may take up to a 12% earnings hit in 2017.

But the real story here has nothing to do with Wal-Mart itself.

It's the change in the market's flow that's important for investors.