Classified reports. This technique is called the Canary Trap, with the name (but not the technique) coined by Tom Clancy.

https://en.wikipedia.org/wiki/Canary_trap

For that purpose you'd want to add typos that would be "corrected" into the wrong word by a typical spellchecker or that are actually correct, but atypical in context, words. Otherwise passing the document through a spellchecker would remove the watermark.

Heh, perhaps an easier way would be to have unique phrases in each document. Typos risk exposing you because they will stand out to the person who is reading the document - assuming such a person is more discerning than most, given that they're accessing classified docs.

Okay, If you broaden it enough I'm sure you can find some use for almost everything. I was thinking of more of a mainstream application.

Pre-publish review copies of a novel