The US stock-market crash associated with portfolio insurance and computerized selling was Black Monday, the 19 October 1987 collapse.
Portfolio insurance was designed to limit losses by selling futures or other assets as markets declined. Alongside computerized trading and a large volume of sell orders, these strategies may have amplified an already weakening market. The exact causes remain debated, because no single factor fully explains the speed and scale of the fall.
The Dow Jones Industrial Average dropped 22.6% that day, its largest one-day percentage decline. The selling spread internationally, although different markets experienced different timing and losses. Trading infrastructure also struggled with the extraordinary volume.
Black Monday is not the same as the 2010 Flash Crash. The 1987 event unfolded over a trading day and involved a global market collapse, while the 2010 episode featured an exceptionally rapid plunge and rebound. Post-1987 reforms included market-wide trading halts, commonly called circuit breakers.