I just noticed that Bitcoin today dropped by an extraordinary -5.65σ — this is only the fourth such extreme event in its entire history since 2010. Theoretically, this should not have happened at all; the probability is one in a hundred million under a normal distribution. But in practice, markets are structured differently, and σ-events occur more often than statistics suggest. I remember that such events only happened before the flash crash in 2020. Interestingly, quantitative strategies caught this move again — even with a low leverage of about 1.4x, the drawdown reached 30%. This is a good reminder that no models fully hedge against tail risks. Risk management systems need to be reevaluated.