From a post: Photographic Mystery Solved by Gerald C Koch on the Analog Photographer’s User Group (APUG). Video from YouTube.

The British photographer Roger Fenton (1819 – 1869) was noted for his photographs of the Crimean War. He is the subject of a recent very interesting PBS radio special. One of his most famous photographs is titled “The Valley of the Shadow of Death.” He has been accused of faking this photograph by making changes to the scene. Susan Sonntag believed that the scene had been manipulated. Here’s where the mystery enters the debate. Another nearly identical photo exists showing no cannon balls on the road. An analysis of both photos shows that they were taken on the same day within hours of each other. Obviously the scene has been altered but which photo is the real one? Conventional analysis of shadows had failed to ascertain which one is the real one. The mystery remained unsolved for many years until recently. Under high magnification a crucial difference was noted, that is five pebbles had moved. Since pebbles do not roll uphill it became obvious which photo was taken first. The famous photograph had been faked by placing cannon balls on the road for a more dramatic effect.

The first photograph from the site: Crimean War Photographs by Roger Fenton, March-June 1855 shows the photograph in question. There are many more interesting photographs.