Isn't this a standard method for a 'poor-mans copywrite' in cartography?
However putting a non-existent island on a Marine chart used for navigation is a few steps above that. It looks like it's the result of a mistake, a mistake that was copied far and wide.
These are also referred to as mountweasel's or 'nihil articles' (http://www.omniglot.com/blog/?p=6187) and turn up in dictionaries, maps, charts and other reference works.
See also: http://en.wikipedia.org/wiki/Fictitious_entry for a long list, including fictitious german politician Jakob Maria Mierscheid in most official parliamentary listings.
just a misunderstanding, I was searching my brain for the phrases you mention and out popped 'poor mans copyright'[1]. You are indeed correct this is a totally different thing and "Mountweasels" (fictitious artifacts) are a very sound method of establishing the originality of certain work... I have to wonder if "Mountweasels" hasn't krept into our language in just such a manner though!
'Copyright', as it governs the right to copy.