It all started on a hot spring day in Barcelona. I waited for the subway to arrive, and spotted this beautifully rugged shaft. First, I shot it with my phone*, then realized I still had time for a "proper" shot and took another picture with my camera. That's when I realized that, in comparison, the photo from the iPhone 4 showed a large green spot in the middle, and slightly pink corners.
Once the subway had arrived, I browsed through my photo collection - and the realization was stunning: green blobs everywhere!
In some shots, the effect was subtle, in some, extremely striking (as in: every shot of a white screen). And it wasn't just my phone: Green blobs are all over flickr, too.
So, now that I've trained you to see green blobs in all of your photos (hehe), I'll also provide an easy way to remove them.
Take this (right click and "save link as") inverted green blob, and throw it, together with your photo, into your favorite** image manipulation suite. The blob becomes the top layer, and should be set to "overlay" (or, in lack thereof, "multiply" will also do). Immediately, you'll get a hue-corrected, and much more natural picture.
*not without being yelled at by a rude construction worker, for taking "illegal" photos. People in Barcelona are less friendly and helpful than in any city I've ever been - and that's from a German's point of view! My photos survived just because the offended guy stood on the opposite platform.
** If you're even more serious about this problem than I am, you could use Imagemagick to correct all your photos with a single batch command. If you already did that, lease leave a photo of your beard in the comments.