- on a forum, if the post reads slightly different to each user, and one user “quotes” it, the quote will be of what that user saw, and other users will be able to identify the fingerprint from the quote.

- if the fingerprinting script modifies all posts on the forum, then a poster will be alarmed to see his words change.

There are clever ways around both of these issues (rewriting the quotes on render, and hiding the fingerprinting from the OP). But eventually the system gets pretty complicated, and ultimately you’re visibly presenting different text to different users, so it’s no longer an invisible fingerprint.