`last_editor` will be missing (empty) from the results when the last editor is a deleted user.  This deleted user does not have to be the owner, or original poster, of a given question or answer.

For example, see [this answer][1] -- which [looks like this in the API][2]:

    {
      "owner": {
        "user_id": 362168,
        "user_type": "registered",
        "display_name": "Saqueib"
      },
      "is_accepted": true,
      "score": 1,
      "last_activity_date": 1416395361,
      /*-- last_editor is enabled in the filter, but not returned because 
        the user was deleted.
      */
      "last_edit_date": 1416395361,
      "creation_date": 1416390599,
      "answer_id": 27013671,
      "question_id": 27013595
    }

The owner is user 362168, while the last editor is a deleted user.

For more examples, see [this edit of Sean Allred's SEDE query][3].


----------
In the case of multiple deleted users, or to determine more information, use [the `/posts/{ids}/revisions` route][4].  Which [in this case gives][5]:

    {
      "user": {
        "user_type": "does_not_exist",
        "display_name": "user4269260"
      },
      ...
      "post_id": 27013671,
      "revision_type": "single_user",
      "revision_number": 5,
      "comment": "syntax edits",
      "revision_guid": "6C1F8C97-615C-4834-B9C5-881CB3F166E5"
    },
    {
      "user": {
        "user_id": 362168,
        "user_type": "registered",
        "display_name": "Saqueib"
      },
      ...
      "post_id": 27013671,
      "revision_type": "single_user",
      "revision_number": 4,
      "comment": "added 33 characters in body",
      "revision_guid": "C55616E3-4467-4240-8781-8ED91D006C1D"
    },

Note that for revision_number 5, `user_id` is not present because deleted user's ids are removed (or hidden from us plebes).

It is for this same reason that `last_editor` is missing, because that is of type `user_id`.

  [1]: http://stackoverflow.com/a/27013671/331508
  [2]: https://api.stackexchange.com/docs/answers-by-ids#ids=27013671&filter=!SV_QVUCYzkf9gbTh0o&site=stackoverflow&run=true
  [3]: http://data.stackexchange.com/stackoverflow/query/251228/
  [4]: https://api.stackexchange.com/docs/revisions-by-ids
  [5]: https://api.stackexchange.com/docs/revisions-by-ids#ids=27013671&filter=!6J*A0qB17gtfb&site=stackoverflow&run=true