Robert Rubin, the former Treasury Secretary and ex-Goldman Sachs chairman, just dropped a warning worth paying attention to. His message? Don't forget what happened during the October 1987 Black Monday crash.

For those who weren't around back then, Black Monday saw the Dow Jones plummet 22.6% in a single day—the largest one-day percentage decline in stock market history. It was brutal, swift, and caught most investors completely off guard.

Rubin's bringing this up now isn't random. Markets have been riding high, valuations are stretched in certain sectors, and complacency tends to creep in during extended bull runs. His point seems clear: history has a way of repeating itself when people least expect it.

What made 1987 particularly nasty was the combination of programmed trading, portfolio insurance strategies gone wrong, and a lack of circuit breakers that exist today. While market structure has evolved since then, the underlying psychology—fear, panic, herd behavior—remains unchanged.

Whether you're heavily positioned in crypto, equities, or both, veterans like Rubin reminding us of past crashes serves as a reality check. Nobody's saying a meltdown is imminent, but ignoring historical precedent? That's how portfolios get wrecked.