I vaguely recall some secretive company (perhaps Apple) using adjustment of spacing, capitalization, etc. to encode a unique serial number in messages sent by the CEO, which could then be used to trace leaks.
It's a plot point in a Patriot Games, a 1987 Tom Clancy novel that introduced the term "canary trap" for this trick. He says he invented the term, but not the technique, which was already in use.
In a spat over the plot of Star Trek III (so, early 1980s), Harve Bennett distributed slightly different versions of the script, allowing him to track a leak back to Gene Roddenberry.
The book SpyCatcher says it was in routine use at MI-5, and you can find variations of it in lots of fiction too.