Key Loss
Permanently losing the secret needed to move your assets, which no company, network, or court can restore.
Ownership on a public blockchain means possession of a key, so losing it removes the ability to spend even though the balance remains visible to everyone forever. The common paths are a recovery phrase written once and misplaced, a device that fails with no backup, a passphrase remembered incorrectly, a backup stored in an online account that is later closed, and death without instructions anyone can follow. Mitigations trade convenience for durability: recovery phrases stamped in metal, copies kept in separate physical locations, multi-signature arrangements where losing one key is survivable, threshold schemes that split a key into shares, and documented inheritance instructions held with a lawyer. A meaningful quantity of early bitcoin sits in addresses that have never spent, which is generally attributed to keys lost in the network's first years.
In practice
A two-of-three multi-signature setup with keys in three separate locations survives the loss of any one of them, which a single recovery phrase does not.
The common misunderstanding
That an exchange or the network operators can reset access, when there is no administrator and a key is not a password stored on anyone's server.