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1\section{Conclusion}2\label{sec:conclusion}34Three decades of minute-level data deliver a sharper statement of intraday5market fragility than either the efficiency or the fragility narrative alone.6Isolated idiosyncratic shocks---even six-sigma events in the largest five7hundred securities---do not propagate: their abnormal cross-sectional8footprint is statistically and economically zero at every horizon from one9minute to two hours, in every decade. What changed is the market's handling of10common variation: information that in the mid-1990s took half an hour to11appear in cross-stock comovement now arrives within the minute, ordinary12extreme moves became an order of magnitude rarer, and extreme synchronization13concentrated in systemic co-jump minutes whose frequency explodes in stress14episodes. Fragility migrated from securities to minutes.1516The evidence is deliberately associational where identification runs out:17OHLCV bars do not observe quotes, depth, or order flow, so liquidity operates18through proxies; tickers are not permanent identities, so long-run statements19run within screened spells; and the co-jump series conflates common news with20endogenous synchronization---separating the two requires timestamped news and21quote data, which we leave as the natural extension. For market design and22risk monitoring, the practical implication is direct: surveillance aimed at23single-name events monitors the part of the system that modern markets already24absorb; the binding constraint is the market-wide minute.25