After winning an auction, the envelope I received the stamps in had a nice color shift to a block of "Save our Cities,Air, Soil, Water" stamps.
The cover actually has a regular block at upper/left, and the color shift block at bottom/right.


The shift gives the fish a blue fin on the bottom and yellow trailing all other fins.

In the Cities stamp, it causes a rainbow of colors to the buildings, and doubles the slide, changing it from white to yellow and a blue double!
So all this was a bonus with the auction. Don't know how common it is or it's value, but it sure made my day.