Discovered Brainwallets

Brainwallets are cryptocurrency wallets with a private key that is directly derived from a human-chosen passphrase, instead of key entropy generated from a (potentially weak) random number generator. Additionally, the key derivation method is usually extremely fast, such as a single round of a very fast hash algorithm operation.

This design makes brainwallets extremely susceptible to offline brute-force attacks, so they're a great way to lose cryptocurrencies to attackers.