Coinsandcanada.com

Looks like an ink error. Possibly something to do with the signature? I took the picture with a black piece of construction paper behind it so there is no light going through the bill.

Honestly under the microscope it looks like an inverted signature on the wrong side of the bill.

Does anyone think it's possible that it is ink transfer from when the sheets are stacked before they are cut

It looks more like an ink smear. An offset would show the face of the note in more detail.
Hard to say exactly without the bill in hand.