Post notices are short text boxes that are attached to posts in order to annotate some exceptional situation. They allow ♦ moderators or bounty donors to let other users know something about the post outside of the normal channels of edits, votes, comments and answers.
Notifications are not intended to replace the normal channels, but to annotate exceptional situations. Therefore, it's unlikely that a post will actually need an annotation. Comments, votes, edits, and new answers may be better choices. However, if you do think a post needs to be annotated by a moderator, feel free to raise a flag. (A word of caution: moderators will not be amused if you flag every answer you disagree with. Exercise restraint.)
With the exception of bounties and some locked posts, the notification will remain until it is removed by a ♦ moderator. So if you think a notification should be removed, the best bet is to fix any problems and raise a flag:
The notices are designed to be self-explanatory and provide relevant information. However, since you asked, I'll do my best to provide the "inside scoop":
There are essentially 6-types of post notices. Each has its own meaning:
This question's answer is a collaborative effort: if you see something that can be improved, just edit to improve it! No additional answers can be added here
Appears to be added to old questions on Stack Overflow that used to have lots of "sub answers" that are now merged to single community wiki post.
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.
Most common on Skeptics, this means the answer falls short of the site's criteria for citing sources. The best remedy is to edit the answer to included the requisite sources.
Post is related to a rapidly changing event.
I've never seen one of these in the wild and the meaning is unclear outside of it's original home (also Skeptics). Presumably the only thing to do about it is to ask in the comments.
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.
We appreciate the desire to help, but please consider either expanding your answer or deleting it. Most answers with this notification are really better suited to be comments in their current state.
This question has an open bounty worth +X reputation from [user] ending in [time].
This question had a bounty worth +X reputation from [user] that ended [time] ago; grace period ends in 23 hours
Congratulations! Someone else cares about this question enough to donate their hard-won reputation in order to get it answered. Enjoy!
There are a number of reasons for issuing a bounty, but they are functionally the same.
This post has been locked due to the high amount of off-topic comments generated. For extended discussions, please use chat.
For whatever reason, people have used this post to basically chat about something other than the post. These locks are usually timed to expire after a certain period of time.
This post has been locked while disputes about its content are being resolved. For more info visit meta.
Please see: What is a “locked” post?
This question exists because it has historical significance, but it is not considered a good, on-topic question for this site, so please do not use it as evidence that you can ask similar questions here. This question and its answers are frozen and cannot be changed. More info: FAQ.
Please see: What is a historical lock, and what is it used for?
Other question states such as duplicate, migrated, closed, deleted, and protected also include boxes similar to those above, but are not listed among the annotated posts.