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I would have to say that this would not be considered an error on the $20 note. The polymer notes are easily manipulate and replicated certain types of errors once in circulation. After examining the photo, this could be one of them.
For the $50 notes, I would have an issue stating that this was also an error that happened during the printing process, as a simple sharp object could scratch the surface of the note and remove the ink.
If an error can be reproduced easily at home, the error looses credibility.