Monk Island

From WakWiki

Some published maps of Lake Quannapowitt show a small island in the center of the north end of the lake called "Monk Island". The island does not exist, and there is no evidence that such an island ever existed in the past. The most likely explanation for its appearance (although this has not been verified) is that some map publishers protect the copyright of their maps by putting small errors in them, which could be detected if the map were reproduced by another publisher.