Date: 08 Feb 2016
Estimated value: 72 200 000
Early in the morning on August 2, 2016, hackers breached the security systems Bitfinex exchange. As a result, 2072 unauthorized transactions were broadcast on the Bitcoin network, involving 119,754.81216269 BTC in the aggregate . It is unclear how the hacker was able to compromise the multi-signature system, though some suggest the cosigning process may have been overly dependent on automated signoffs on transactions below a certain value threshold.