Modern objects: You do not want modern objects to appear in the image, such as cell phones, computers or cars. This would break immersion and make the image look less realistic.
Defects: You do not want the image to have defects, such as dead pixels, noise or blur. This would make the image look bad and be difficult to enjoy.
Distortion: You do not want the image to be distorted, as this would make it difficult to see details.