Invisible characters cause real, boring damage, and the damage is hard to trace because none of the symptoms point at the cause.
In Word and Google Docs, find-and-replace stops matching a word that is plainly on the page, because what is stored is the first half of the word, then a zero-width space, then the second half. Spell-check flags a correctly spelled word. Your word count disagrees with your editor. Justified text spaces oddly because a non-breaking space refuses to stretch. A citation manager fails to link a reference it should have matched, because the author string contains a character you cannot see.
In WordPress, control characters and DOM attributes copied from the chat window can invalidate a Gutenberg block, so the editor scrambles the formatting on save or refuses to save cleanly at all. Markdown compounds it: the hashes and asterisks publish as literal characters, and a post goes live with formatting syntax around its own subheadings.
In SEO, the failure is quiet and expensive. A zero-width space inside a keyword in a title tag or meta description means the crawler parses a different string from the one you see in your CMS preview. Keyword density reports skew. Exact-match phrases stop matching. Nothing errors, nothing warns you, and the page simply underperforms for a reason that never appears in any audit.
In an LMS or a submission portal, the plain-text box usually strips formatting but keeps characters, so a submission arrives with hashes in front of every heading and invisible bytes scattered through it. Nothing about that is against the rules. It simply looks like what it is.