I just ran across this comment:
First Amendment notwithstanding, there are such things under the law as harmful speech that can be sanctioned.
“Sanction” has two meanings, which are opposites of each other: it can mean either “official permission” or “penalty” (or, as a verb, either “to give official permission” or “to impose a penalty”). In this case, since the topic is the first amendment, which permits people to say things that have historically been banned, it’s even more confusing.
David at Mental Floss calls such words “contronyms” or “antagonyms”. Take your pick.