A teacher may answer a question or make a personal remark before beginning a hadith and its chain. A pupil who does not detect the transition may treat the teacher's preliminary words as though they were part of the report carried by that isnād.
This case is presented within the discussion of isnād interpolation because the distinction between the teacher’s speech and the material formally transmitted through a chain has been blurred. The result may also appear as an intrusion into the matn.
Contrast: “The teacher commented that nighttime devotion is beneficial, then recited a hadith through his chain.” These are separate acts. If both are retold as one prophetic report, other pupils’ versions of the same session may expose the boundary.