Mapmakers insert incorrect entries to see when someone illegal copies their work, they are called "copyright traps"

Fictitious entries, also known as fake entries, Mountweazels, and Nihilartikels, are deliberately incorrect entries or articles in reference works such as dictionaries, encyclopedias, maps and directories. Entries in reference works normally originate from a reliable external source, but no such source exists for a fictitious entry.

Fictitious entries can be humorous hoaxes intended to be more or less quickly recognized as false by the reader or copyright traps deliberately inserted into a work to facilitate detection of copyright infringement or plagiarism.

http://en.wikipedia.org/wiki/Fictitious_entry