During the second phase of processing, the database history is queried to build a chain of backup images required to perform the requested restore operation. If, for some reason, this is not possible, and DB2 is unable to build a complete chain of required images, the restore operation terminates, and an error message is returned. In this case, an automatic incremental restore will not be possible, and you will have issue the RESTORE DATABASE command with the INCREMENTAL ABORT option. This will cleanup any remaining resources so that you can proceed with a manual incremental restore.
During the third phase of processing, DB2 will restore each of the remaining backup images in the generated chain. If an error occurs during this phase, you will have to issue the RESTORE DATABASE command with the INCREMENTAL ABORT option to cleanup any remaining resources. You will then have to determine if the error can be resolved before you re-issue the RESTORE command or attempt the manual incremental restore again.