Document label mismatches happen when users create, upload, or update Office documents in SharePoint sites and give the documents a higher-priority sensitivity label than the one assigned to the site. When this happens, SharePoint Online creates a DocumentSensitivityMismatchDetected audit event. Unhappily, that event doesn’t tell us who caused the mismatch, but some work with PowerShell reveals all.
Support for sensitivity labels is generally available for SharePoint Online. Users can apply labels to classify and protect documents, but a mismatch can happen between labels applied to documents and the sites where the documents are stored. When this happens, SharePoint Online emails site owners to tell them that a mismatch exists.