I noticed a minor bug today. If a question has a post notice, that notice disappears if any of the answers (or the question itself) are refreshed. By "refreshed" I mean by clicking on the little gray bar at the top of the post that says something like "this post has changed... click this to refresh it".
Here is sample post, with the notice:
And this is what happened when I refreshed the answer post:
I noticed this on MSO just shortly prior; I also noticed this behavior on SO with a question close notice (not 100% sure on the latter though).
I should add that the notice disappearing is temporary. It comes back when the entire page is reloaded (e.g., pressing F5).