In our seminar today we discussed "Exposing Photo Manipulation with Inconsistent Shadows" by Eric Kee, James F. O'Brien, and Hany Farid. After the presentation I showed an extra example I'd witnessed in an advertisement on the Zurich Tram.
Our drivers make this trip over 90 times a year.
The crux of "Exposing Photo Manipulation with Inconsistent Shadows" is that if you identify enough correspondences between points on shadows silhouettes and objects that may have cast them, then you can determine a feasible region where the light source is. Any correspondences that are inconsistent with the others are evidence of image forgery.
So I walked through the algorithm on this image marking plausible shadow correspondences as wedges or half-planes. The intersection of the consistent ones form a non-empty feasible region for the light source. The suspicious shadow of the tram sign is inconsistent as it does not intersect with that region.
In my searching for the original ad, I also found this album cover.
It uses the same image of the moon! Looks like the tuba's shadows are a bit more realistic that the tram sign's. Maybe there's a tuba on the moon?