VIX held the key to the heads up for a cliff hanger...

CBOE:VIX   Volatility S&P 500 Index
Seen here is the VIX chart, and notably, two segments are highlighted.

First the Green box where VIX dropped to 21 as SPX rose higher.
However, later on, in the Red box area, a siumilar rise to the same level was not concomitantly seen with VIX retracement. In fact, the VIX was resilient in holding its ground at a higher level.

Subtle, but very significant.

A good learning point and we should see more of such... usually the downside moves are significant under these circumstances.