The open-ended prophesy is a favorite trick (among many) of psychics, astrologers and clairvoyants.
There are two famous examples of this:
(1) The answer the Delphic Oracle gives Crœsus when he asks about attacking the Persians. The Oracle said:"If you attack you will destroy a mighty empire." Crœsus attacked, but destroyed his own empire.
(2) The reply Rivka receives when she "she went to enquire of the Lord" She is told "...and the elder shall serve the younger," in Hebrew וְרַב יַעֲבֹד צָעִיר. As Robert Alter says, the Hebrew syntax leaves unclear which noun is subject and which is object -- "the elder shall serve the younger" or "the elder, the younger shall serve."