The Mt. Gox collapse (Feb 2014)

In February 2014, Mt. Gox — the Tokyo exchange that had handled the large majority of all bitcoin trading — suspended withdrawals and filed for bankruptcy, reporting roughly 850,000 BTC missing (about 140,000 were later recovered). Years of litigation and a creditor process followed that only began paying out a decade later.

Gox taught the industry its foundational lesson at maximum cost: an exchange balance is a claim on a company, not bitcoin. “Not your keys, not your coins” went from slogan to survivorship rule — and every custody collapse since has rhymed with it.