A typo in the Zcoin source code allowed an attacker to steal 370,000 Zerocoin, which is about $592,000 at today’s price. One extra character left in the source code caused a bug that an unknown attacker discovered and used to his advantage in the last few weeks. The Zcoin team says the attacker (or attackers) was very sophisticated and took great care to hide his tracks. Zcoin is the precursor of Zcash and Monero, two similar cryptocurrencies that provide extra anonymity.
Source: https://www.bleepingcomputer.com/news/security/a-source-code-typo-allowed-an-attacker-to-steal-370-000-zerocoin-592-000-/