A new feature has been introduced this summer, tag warnings.
These warnings are meant to provide users with just-in-time information to help them avoid common mistakes in the tag they’re using (e.g. SQL questions should mention the engine they use).
Here’s what they look like:
(More information on this here, from which the above text and image were taken.)
It seems as if it could be a really helpful feature, especially for new users, so they can avoid common pitfalls. Indeed most people seem not to read tag excerpts, but they might read a yellow warning popup. A few examples of tags for which it could be helpful are physics, computer-algebra-systems and its friends, soft-question, logic (as opposed to elementary-logic) and similar cases, publishing, extremely general tags like number, etc. See also this related discussion about "off-topic tags".
I suggest that we follow the format laid out at meta.academia.SE: write down the tag name, the proposed warning text in a block quote, and some context.