Unplanned downtime can be classified into downtime caused by system faults, data and media errors, and site outages.
System faults can occur in the system itself (CPU, memory, or power supply), the software (operating system, database, middleware, or application), peripherals, power, or the network.
Data errors include human error by DBAs, system administrators, users, and operators, as well as sabotage. Media errors include disk failure and data corruption.
Site outages include natural disasters, power failure, and sabotage.