A old article I just ran into in the New Yorker: Not a word, the interesting story of Lillian Virginia Mountweazel, an American fountain designer turned photographer who died prematurely in an explosion while on assignment for Combustibles magazine.
The interesting part is that she didn't exist, but appears in the 1975 New Columbia Encyclopedia as a copyright trap: if her biography turns up in any other reference work, the publishers will know that it has been stolen rather than independently researched. Another example cited is the appearance of a non-word, "esquivalience", in the New Oxford American Dictionary. It's nice to see this phenomenon confirmed, as it has rather an urban myth flavour to it.
As you can gather from the Wikipedia Mountweazel entry, fictitious entries have varied motives: copyright traps, plain mistakes, hoaxes for fun, and hoaxes for fraud. For instance, the 1880s American Appleton's Cyclopedia of American Biography has around 200 known hoax entries. In this case, the probable motivation was financial; the contributors being paid by space and with checking only for general style, the temptation must have been too great.
Similar considerations apply to maps. In 1999 this Telegraph article, AA in £20m battle over 'copied' Ordnance maps, reported how the Automobile Association was caught out as having copied from Ordnance Survey maps a number of deliberate stylistic fingerprints - "kinks in rivers, the addition of minor buildings or exaggerated curves in roads". You can find other examples at the OpenStreetMap article Copyright Easter Eggs - Maps that Lye, and confirm using Google Maps that there are definite discrepancies between the street data and aerial photos, such as The Avenue, Finchley, that's displaced to cut straight through a block of flats. As with textual references, it's a matter of opinion whether many of these are deliberately fake "trap streets", simple errata, or time-related errors (such as road layouts revised by subsequent building, or reliably anticipated ones that never came to be). - Ray