All of the following are considered pre-analytical errors EXCEPT:

Pre-analytical errors refer to mistakes that occur before the actual analysis or testing of samples in a laboratory setting. These errors can arise during various stages including patient preparation, sample collection, handling, and processing.

Ordering a lab test under an incorrect patient number involves issues with identification that can lead to mismanagement of patient samples. Misidentification of a patient is a critical error that directly affects the integrity of the testing process and ultimately the safety of the patient, as it can lead to incorrect diagnosis and treatment. Mislabeling an add-on test similarly impacts the reliability of the sample, as the test may not be accurately attributed to the correct patient.

On the other hand, a transcription error entering a manual result occurs after the sample has been analyzed, making it an analytical or post-analytical error rather than a pre-analytical one. This distinction is significant, as it highlights where the error takes place in the testing process. Therefore, the correct choice identifies an error that falls outside the pre-analytical stage, emphasizing the importance of proper data management as a separate step after the analytical processes are complete.