I bet you are right Matt and that this error is more common than we care to admit. Unlike upside down or missing serial #'s this could easily go undetected by most people. Looking forward to what PMG says.
There are many 1996 $100's with this error. The currency paper is manufactured with a pattern of notches along the edges of the sheet, different for each denomination; the idea is that it'll be really obvious if a sheet is facing the wrong direction, or mixed in with sheets intended for another denomination. Unfortunately, a large batch of $100 paper was delivered to the BEP with the notches at the wrong end, so quite a few $100's were printed with inverted watermarks. The AB..D block seems to be the most common, but there are others too, printed at around the same time. I haven't heard of this error occurring on any other denomination besides $100's. Doesn't mean it hasn't happened, but I'm pretty sure it hasn't happened in large quantities, at least. Azpiazu has an example of the 1996 $100 with inverted watermark in AU, offered at $375. (No image of the note, unfortunately.)