Recently, it was discovered (or undiscovered) that an island that had been included on maps for years did not exist. The story broke recently and even made it onto the BBC web site, and since then people have been trying to track down the history of this fictional island.
The blog at Auckland Museum has a chart that places its discovery in 1876 to a ship called Velocity, but the blog notes that the map came with a disclaimer warning readers of the accuracy of the positions of low-lying islands in that part of the ocean.
As a member of the army or armchair experts created by the internet, I set off on a voyage of discovery to find traces of this island for myself. It didn't take long to realise I was already walking in other people's footsteps as I found little new on the subject. I did, however, find on Google Books a volume dated 1851 that listed Sandy Island (New Caledonia) in its index.
So I was all excited that I'd found something that everyone else had missed, until I looked at the entry in closer detail. Unfortunately, this refers to a smaller Sandy Island which is much nearer the coast of New Caledonia and does, to my knowledge, really exist.
But, anyway, this is a beautiful story and it's a useful reminder that however important and established an authority may be on a subject, it doesn't hurt to check the basics.
Perhaps the fake Sandy Island kept itself in reference books simply by being in a place where no one would check, and by having a name identical to another island not so far away. You know, when I was growing up in Hertfordshire, I would sometimes see signs to a village called Ireland. I wished I'd checked if it was really there, and not some cartographer's quirk.