The problem I'm starting to have with SCF is that every time someone points out a possible flaw, or something unusual in a stamp, I've got to go running to my stock pile and rifle through to see if I can find something similar. I think I need help.
Anyway, that aside, I think colour shift, or mis-registration of two colours is pretty common, especially when you are dealing with such small pieces of art. The plates are bound to move a little bit.
I found three examples of this stamp with slight shifts in registration, but what I thought interesting was the actual difference in colour between the stamps.

The top two in this picture are perf. 12 (1938-51, exact date not specified). The bottom is perf. 13.5x13 which Scott's dates from 1950 and shows a much lighter colour variation, especially in the red violet.
Nothing earth shattering, but I had to make the trip to the hoard worth while.
