A special way of watermark
ing a document to tighten leaks by espionage and the like.
Lets say the American government creates out a secret report on the situation in South America. They make dozens, nah, more like hundreds of copies for cabinet members and high ranking officials in the necessary branches and military leaders. What if this somehow gets into the hands of the press? How will they know who to fire for spilling the beans? It's called a canary trap to catch whoever "sings."
The way this countermeasure works is simple. For each copy of the document, change the punctuation in some way. Omit a comma, change one to a semicolon. Do this so that every copy of the document is unique.
So if the next day the New York Times publishes a photograph of a leaked presidential memo about clandestine troop movements, they can figure out and trace which copy was leaked. Clever, eh?
Lately the reporters have gotten smarter. Shows such as Dateline don't really show the document, they just quote it. In The Hunt For Red October, Jack Ryan developed a new method for the government in which instead of punctuation, synonyms were put in. The new words were more...lurid. The idea is tempt the media to quote the excerpt verbatim for a juicier story, thus tracing the origin.
It's not just the government, Hollywood studios use the canary trap with scripts, private companies with patent ideas as well. It's also an old mapmaker's trick, add a fake road or river here and there so if they are duplicatd, you can prove somebody stole your work. I've also seen somebody do this in Computer Science class to prove he was being plagarized.
Movies are different in methodology. Each theatrical print (35mm film) has an individual serial number and each print is somehow slightly different than any of the others. Therefore, when a print gets hijacked and pirated, the studio can tell which print serial number it is, and can then trace that print back through all of the theatres where it has been played, theoretically being able to isolate the "culprit" who leaked that print.
Also, the idea of watermarking CD's has been lobbied for years by the RIAA. The canary trap for CD's is pretty ineffective; it's possible, but not worth the trouble. Someone on slashdot argued that the introduction of a song could theoretically be changed, so that leaked copies from radio stations and ripped songs could be traced. That isn't feasible, as you could either detect or hear a change in the audio, then cut it out. Burying it in a header of the sing file will disappear as you convert formats, like .wav to .mp3. Plus, all radio stations are bound by contract to receive identical copies, anything else is considered a remix of the original and grounds for a lawsuit.
Aside from that, each CD would then have to be separately burned, as they are no longer identical, taking much, much, much longer than pressing a mass amount of albums.