This problem has been bugging me on and off for more than 10 years (I should've asked earlier I know).
This occurs when someone opens a CMYK image with a software that does not support CMYK color space (for example, older versions of ACDSee), and saves the image into RGB. It will result in a strange color cast. (like the image below in the top)
My question is how do you restore the original, correct color (the bottom image)? I have tried with complex curve adjustment with close result, but i'm wondering if there is some curve that is "standard" and can be quickly applied to this kind of images.