When Merrick removes the lens cap while taking a photo of the cavalry officers; he loses his grip of the lens cap and it remains in front of the lens. Since photographs taken in this way required a long exposure a mistake like this would have resulted in ruined picture. The picture that Merrick later laments about and hopes to re-shoot shows no evidence of the lens cap that was in the way earlier.
When General Crook is telling E.B. that he should work with Captain Bubb for the troops "logistical needs", he points at the Captain with his left hand across his body. When the camera angle changes to behind the General, he is pointing at Captain Bubb with his right arm extended straight out.
In the first scene, after Dan hands Al his morning coffee, Dan walks under the shadow of silk screen frame, a piece of lighting equipment used to diffuse the sunlight. The tell-tale outline of a C-stand adjustment screw passes over his chest.