When a global People tag suggestion for a photo or video comes from Windows Live SkyDrive or Facebook, and is presented in Batch people tagging mode as a suggestion for a person, the thumbnail shown doesn't appear as expected. For photos, the thumbnail is not square (it has the same aspect ratio as the original image), and for videos, gray thumbnails appear.
Also, if there's more than one face in the photo, there might be confusion about who in the photo is being tagged.