UniqueFieldValueValidator validates the UniqueFieldConstraint; it is supposed to ensure that a field value is unique, that it cannot have the same value on multiple entities of the same entity type.
Previously, a bug in this validator meant that it only considered entities that the current user had view access to. A field on an entity would have passed validation with this constraint if the same value occurred in the field on a different entity, and the current user did not have access to that entity.
This bug has now been fixed. As a consequence of this, it is possible in rare cases for some existing entities in the database to be in an invalid state; in such a case, if a content editor opens a form to edit the entity then the editor will not be able to save the entity if they make no changes to it. If the editor is not able to change the unique field value, then the editor will be unable to save the entity or make any changes to it. In such a case, developer intervention will be required to set the field to a valid value programatically using the Entity API.
