Hi Manal,
Interesting and I do have a similar note in my collection. If someone would consider this an error, it would be a very very very minor error.
As to how this occurred, it is tough to say;
- The numbering mechanism could have been over inked and a lnk droplet fell onto the note
- The numbering wheel could have a piece of debris in the mechanism that left a small dot on the note
- A spring in the mechanism got loses, acting as an other surface to grad the ink.
Hope this helps