The S&P 500's cyclically adjusted price-to-earnings (CAPE) ratio, also known as the Shiller P/E, reached 40.7 as of September 11. This marks its highest level since the dot-com bubble, when it peaked at 44.2 in November 1999. The Motley Fool analysis highlights that while the current run is fueled by the AI boom, this valuation level suggests a historical lesson for investors to prepare for an eventual market pullback.