I might well have read it here or on reddit, I don't remember.

I guess it's ok to propagate a factoid as long as it's true.

I got bit by this once: a factoid is an anecdote that people share thinking it is true, but it is not. From reading your comment, I think you just meant "... ok to propagate a fact as long as it's true.". Just a friendly heads-up :)

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

Your Wikipedia link then continues 'the term has evolved from its original meaning, in common usage, and has assumed other meanings, particularly being used to describe a brief or trivial item of news or information."

We no longer use factoid in that meaning. Even your clarification here is not really how Mailer defined it; as "facts which have no existence before appearing in a magazine or newspaper, creations which are not so much lies as a product to manipulate emotion in the Silent Majority". (See http://www.etymonline.com/index.php?term=factoid ).

It's on the same path that "computer" (someone who computes), "awesome" ("profoundly reverential") and many other words have been on.

And the same path of "couldn't care less" versus "could care less."

Does this apply to legal language?

I have no idea. How do you mean?

I was wondering what happens when the popular meaning of a word or phrase diverges from a law which uses that phrase for a different meaning or scope.

What happens is, things get complicated.

For a related example, consider the phrase "we settled the debt without prejudice" That sounds like it means there are no bad feelings, but it actually means that there may be future attempts to change the terms of the negotiation.