This stamp completely freaks me out.
If you look at the image of the complete stamp below, you'll probably assume, as I did when I first saw the stamp, that the corner of a different stamp has been glued onto the corner of this one. It looks like a very crude effort has been made to repair a torn corner:

But this impression is completely false. There is no join. The surface of the stamp is one seamless whole. If you run your finger over the area where the join seems to be, there is not the least disturbance. In this area, the stamps feels as smooth as any other part of the stamp's surface.
How this could be possible, I have absolutely no idea.
If this is not how the stamp left the printer's office, it is a piece of philatelic illusionism worthy of an appearance on 'Fool Penn & Teller'!
Here's a detail for those who may be intrigued:

Explanations for this mystery are, of course, extremely welcome!