It was old tape practice that in case of a write error, a longer gap was 
created to pass that area.
Although i do not know exactly how tape drives are controlled today, this 
procedure should still be implemented (in their firmware).
This totally transparent to the user like handling a bad sector on disk.
So many write errors will affect the tape capacity as well.

NW will only assume a tape error and set the media to full if it has 
encountered 20 (default) CONSECUTIVE write errors.

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