Final answer:
Underexposure during film development causes black, crescent-shaped marks on a photo due to too little light reaching the camera's sensor or film.
Step-by-step explanation:
The processing error that causes black, crescent-shaped marks on a photograph is underexposure during film development. Underexposure occurs when there is too little light reaching the camera's sensor or film, resulting in dark or black areas in the photo. This can happen if the shutter speed is too fast and/or the aperture is too small. In such cases, the parts of the photo that should be properly exposed appear dark and may have crescent-shaped marks.