Copyright Easter Eggs

I was reading up on the OpenStreetMap project and other GeoWikis, when I discovered this page describing something known as Copyright Easter Eggs.

In order to defend the copyright ownership of one's map data, some map manufacturers have inserted small errors into the geodata so that if anyone was to copy the copyrighted maps, they could show that the copy was derived from their copyrighted source. Examples of this are streets that don't exists, names of streets that are slightly altered or misspelled. Churches that don't exist are also popular fake errors.

It's an interesting application of digital watermarking. Although the errors are not imperceptible (the errors are easily spotted if the subject viewing the map simply knows what to look for), the fact that the errors are fairly random and interspersed with actual accurate data and that there is no easy way to find all of these introduced errors, it makes it a seemingly fairly effective way to demonstrate copyright.