A family of Microsoft word processing software products for creating web, email, and print documents.
An original cause of this issue was apparently the inclusion of a .gif image file in a document and that was addressed in an earlier update. It has however raised its head again and in one case that I am aware of it is due to the presence of an IncludePicture field as part of the argument of a MacroButton field.
This most recent occurrence (within the last couple of weeks) has been reported to the Word Product Team at Microsoft and they are investigating it.
It can be prevented from appearing by setting the DWORD (32-bit) Value of the DisableWarningOnIncludeFieldsUpdate key of the Registry to 1. However, by default, that key does not exist and it is necessary to create it, which is something that many users cannot do because they may not be able to access the Registry due to Group Policy Settings.