A prophecy is a message from God, usually a warning. It is not a prophecy if it does not come true. Thus, a prophecy about a wrong person would not be a prophecy since it was wrong.
Yes - but such a prophet is a false prophet. The Bible teaches that one of the indications of a true prophet of God is that such a prophet will *never* be in error regarding a prophecy. If the prophet said, "this will happen to YOU" and it did not, that is a false prophecy and therefore a false prophet.
Also: the verb is "prophesy" (pronounced "prof-fess-sigh"). "Prophecy" (pronounced "prof-fess-see") is the noun form.