A change was made recently to the display of tag wikis when viewing questions in the tag. Instead of displaying the tag wiki excerpt at the top of the question list, the first section of the full tag wiki is displayed instead.
There are many, many tags that have an excerpt, but no full wiki body. Right now, when the full tag wiki is empty, the following text is displayed:
The foo tag has no wiki summary, can you help us create it?
There is indeed a summary in the excerpt, it's just not being used here. It could be, while retaining the "hey, we need a full wiki for this tag" wording.
I've lately found myself just copying and pasting the excerpt into the full wiki body on a handful of tags, and that strikes me as weird. The excerpt previously displayed with the full wiki as well, which made doing this rather awkward.