Great find. There is a whole field of collecting dedicated to currency errors. Someone with more knowledge will chime in. This one is odd though. It appears as though the "face" side bled through. This would be a fairly significant error and I would think difficult to fake.
2nd print offset error
Offset printed notes occur when an inked plate makes contact with the bed, often the cause of no sheet being fed to the press to accept the intended ink. As a result, when the next sheet passes through the press, the area impressed onto the impression cylinder is then pressed onto the opposite side of the note.
Those overprint errors seem rather popular on ebay and there are some numbers well into the $300+ range. This one was cheaper, and has a very striking overprint. It's worth some $$...