Stocks just had a sharp sell-off, and the warning is that markets worldwide are now moving together so tightly that a crash could come fast.

• A big sell-off hit stocks last Friday, with money rushing into bonds and safety.
• Goldman Sachs warns the real danger is "crowding" — too many people piling into the same trades (AI and commodities) at once.
• Foreign markets are cracking: Korea's index dropped over 8% and triggered a trading halt, and Japan is sinking on currency problems.
• As people lose money overseas, they sell US stocks to cover it, dragging American markets down too.
• The 10-year government bond rate has crested 4.55%, a level that pushes mortgage rates up and weakens housing, America's biggest source of wealth.

Outlook: More downside is expected as global trades unwind, so holding cash to buy cheap later is the play.