cpio: out of sync, bad magic number

Had this appeared on the backup log: “cpio: out of sync, bad magic number”. Not had this before – can anyone tell me if it’s critical and what impact it’s likely to have had on out backup tape?
Backs up to an external AIT. Compaq Proliant Running SCO OpenServer 5.5, 2GB RAM, 250GB RAID 5, The other 2 tape drives backed up with no errors.