Mt. Gox
lorecryptography
The Tokyo exchange that handled most of the world's bitcoin trades until 850,000 of them vanished in 2014.
Born as Magic: The Gathering Online eXchange, Mt. Gox grew into the dominant bitcoin exchange before collapsing into bankruptcy in February 2014, reporting about 850,000 bitcoins missing, most of them customers'; roughly 200,000 were later found in an old wallet. Investigations pointed to years of gradual theft from its hot wallets rather than one dramatic heist. It remains crypto's canonical custody lesson, compressed into four words: not your keys, not your coins.