This is a very tough one, and one that would need to be examined in person due to the fact that the note is in circulated condition and the missing ink is so small. If the note was uncirculated, then you would be able to see the full embossing of the serial number to help determine how this might have happened.
Here are a few thoughts of what could have happened:
- Lack of ink on the number when printed
- The missing portion was obstructed with debris
- The number wheel had a broken/chipped piece in it
- There is a fold in the note through the number and the ink wore off
- Some one took a razor blade and scratched the ink off
Yes this could be a legitimate error, but my concern is that it could also be easily fabricated. With the condition of the notes, it is very hard to say when the missing ink had happened. If the embossing is still very strong, then you may be able to see if the zero was completed with the full embossing and the missing ink, or if the zero is like a backwards C missing the embossing and ink together.
Hope this helps