Not seen this before. I wondered if anyone else had, or had any thoughts on how it happened?
Obvoiusly, stamp is printed first. Then phosphor is added in side bars on top of the ink. Then the cancellation on top of that.
As you can see, some of the blue ink is lost along the way - most noticeably causing the virtual deletion of the dark parts. I'm keeping this, filed under errors, freaks and oddities.