i have to agree with lostdutchman, it looks like a wet transfer error to me too.
here's how it happens:
first, for better visualization, a basic intaglio printing set-up.

now, imagine that for some reason the press is rolling but the paper/felt blanket wasn't there, (someone forgets to load a new sheet/doesn't load it fast enough/doesn't turn it off/etc). the ink from the plate gets transferred onto the top cylinder, which is still rolling. the next time a paper sheet passes through, the ink get imprinted from the cylinder onto the sheet's reverse side, but in mirror image.
here's another example of a note with a wet transfer error.

it's too bad your note circulated that much without somebody rescuing it sooner.