Mods have a new option to add post notices. There are three basic notices currently available but MO specific notices could be added:
citation needed - This post does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed.
current event - Post is related to a rapidly changing event.
insufficient explanation - We're looking for long answers that provide some explanation and context. Don't just give a one-line answer: please explain why you're recommending it as a solution. Answers that don't explain anything will be deleted. See Good Subjective, Bad Subjective for more information.
You can read about the intended usage and history on meta.SO. (I gather the main purpose was for use on Skeptics.)
Should post notices be used on MO? If so, when?