WSJ’s report also contains a few interesting accounts of how recent and upcoming movies were kept under wraps. Each copy of each script for The Hunger Games adaptations has a few slightly different words, so that if it reaches the public the studio will be able to figure out exactly where the leak came from.
It's called a Canary Trap [Wikipedia Link] and it's been used for some years
A canary trap is a method for exposing an information leak by giving different versions of a sensitive document to each of several suspects and seeing which version gets leaked. Special attention is paid to the quality of the prose of the unique language, in the hopes that the suspect will repeat it verbatim in the leak, thereby identifying the version of the document.
As an example...
...before drafts of the screenplay for Star Trek III: The Search for Spock were circulated, Bennett arranged for each individual copy to have subtle clues distinguishing it from the others. Shortly after Roddenberry opposed the destruction of the Enterprise at the climax of that film, fans began to complain to Paramount and Bennett. He found that a leaked copy of the script was the one given to Roddenberry, but was unable to do anything about it.1
From a comment link by @Thunderforge (thanks):
They are sometimes used in television is well. J. Michael Straczynski used them for a key Babylon 5 episode in 1993 to deter spoilers from getting out.
This episode is going to be highly classified; we're going to limit
distribution of scripts, and parts of scripts, put canary traps in all of the
scripts that are distributed, and otherwise keep this one quiet. All I can
say is that we're going to kick over every table we've got. In any season
finale, there are maybe 4-5 things you know when you sit down to watch the
show that they'll NEVER ever do. So we're doing all of them. If this one
doesn't keep you glued to your seats, you've lost your chair.