1. Multiple disk failures beyond the RAID level chosen (e.g. both disks in a mirror, or 3 disks in a RAID-6).
2. Failure of the RAID controller itself (especially when using hardware RAID), the computer itself, or the environment (a flood, or fire, perhaps).
3. Data corruption on-disk (except for ZFS, and especially for BTRFS) from cosmic rays, or minor hardware or firmware failures.
4. File corruption from bad writes or bit rot (except for ZFS), including whole-volume corruption from administrative mistakes (e.g. =mkfs= on an existing filesystem).
5. Malicious or accidental deletion or modification of data by yourself or another party, including viruses.