Study plan deviation means an unintended departure from the study plan after the study initiation date i.e. after the Study Director has approved the study plan.

The OECD Principles of Good Laboratory Practice are clear:

Deviations from the study plan should be described, explained, acknowledged and dated in a timely fashion by the Study Director and maintained with the study raw data

This reiterates the need for regular communication between personnel and the Study Director, so that these changes can be recorded together with any decisions and subsequent actions justified.