My colleague was putting the same Backups (but at different levels: 0,1,2) into 1 tape everyday (i.e. he replaced the tape everyday). He was doing it for quite some time now to save on tapes. When he tried to restore it to another server, it was complaining that it was an invalid archive level. The tape contained Backup Levels 0,1, and 2.
He's using IDS 7.31 on SCO Unix 5.0.5 and the tape device was pointing to /dev/rStp0