For example this video:
If you are describing pain or symptom to a doctor, the interpret cannot "summarize" the state of the patient. This can cause that the doctor have a prognosis to another condition or disease. Also, you as an interpreter, even if you know the patient or the doctor, you cannot answer as if you know the answer, even if you do. You have to interpret everything both people are saying to be clear in the statement.
Links:
Top 5 Mistakes When Interpreting
How to Avoid Mistakes in Medical Interpreting
Nine common errors when Interpreting Biblical Narratives