Final answer:
The mistaken filing of two patients' records together is known as an overlay, which is a term for errors in patient health information systems. An addendum, in contrast, is an intentional update or correction to a medical record.
Step-by-step explanation:
When two patient's records are filed together by mistake, this is commonly referred to as an overlay. An overlay occurs when one patient’s health information is mistakenly placed in another patient’s record. This term is particularly used in electronic health record systems where patient information can be more easily confused if data entry is incorrect, resulting in significant issues regarding privacy and treatment.
An addendum, on the other hand, is a legitimate addition to an existing medical record to update or correct information, or to provide further detail to a prior entry. Addenda are usually clearly marked and dated to make sure that the record remains as accurate and up-to-date as possible without altering the original information.
In summary, the filing of two patients' records together is an overlay, noting a mistake that intertwines distinct medical histories, as opposed to an addendum, which is an intentional update to a medical record.