At the height of the trial in the cinematic court room drama The Verdict, a nurse acting as witness for the plaintiff offers as evidence a photocopy of a hospital admission form showing that the victim of the alleged medical malpractice was known to have eaten just one hour earlier and so should not have been anaesthetized (Lumet 1982). Yet she was anaesthetized, which made her vomit into her face mask, causing brain damage from lack of oxygen. The original admission form shown to the court recorded that the victim ate nine hours earlier (and so could be anaesthetized), but the nurse claimed that she photocopied the form before the anaesthetist (realizing his error) forced her to change the numeral 1 to a 9. On an established legal preference for original documents over photocopies, the jury is instructed to forget it ever heard about the nurse and her photocopy. Happily, the jury ignores this instruction and awards damages against the hospital.
The principle that one should ordinarily prefer an original of something over its copy is central to much of our thinking about textual authenticity, although of course there are circumstances under which it should be set aside, as when one suspects that the original was altered after the copy was taken. If the original was altered, one has to ask why and make a judgement based on one's best attempt at an answer. Originals should normally be preferred to copies because copying introduces errors, some random and some predictable.