President Donald Trump's celebration of Wall Street's highs aged rather poorly. Less than 24 hours after boasting about Americans' retirement accounts going "through the roof," the S&P 500 plunged as much as 3.8% Thursday, bringing emergency trading halts into the conversation. The selloff prompted traders to monitor the market's circuit-breaker thresholds, which can temporarily shut down trading across U.S. stock exchanges when losses become too severe. The S&P 500 would need to fall another 3.2 percentage points from its decline to trigger the first emergency halt.