Original: The UBE (Unrecoverable Bit Error) rate is typically specified at 1 bit in 1015 for enterprise class disk drives (SCSI, FC, SAS), and 1 bit in 1014 for desktop class disk drives (IDE/ATA/PATA, SATA).
Simplified: This happens at about 1 bit in 1015 in enterprise-class disk drives, and 1 bit in 1014 in ordinary disk drives.