Bit Rot
The gradual, silent corruption of stored data over time, in which individual bits change without any error being reported.
In depth
Because the corruption is silent, a damaged file can be dutifully backed up over a good one for years before anyone opens it. The defence is verification: storing checksums when files are archived and periodically re-checking them, or using a file system such as ZFS or Btrfs that verifies and repairs automatically.
Examples in practice
A routine checksum scan finds three corrupted files in an archive, which are then restored from an older backup.