Most untagged tags (such as those on SF, SO) have an excerpt:
This is a special tag that is designed to be used only by the system for questions that have had all of their other tags removed. Do not add this tag to existing questions, it is reserved for use by the system to identify posts with no valid tags.
However, I found at least two sites which miss the excerpt:
Should these excerpts have been added automatically?
If not, can I just add this to the excerpt by suggesting an edit?
You can see from the revisions of the two examples you gave (Stack Overflow & Server Fault) that the excerpt isn't automated but was created (and edited a bunch of times in SO's case) by regular users. Most sites don't have a wiki/excerpt for the non-tag as far as I can see...
So no bug. By all means suggest an edit.
Theoretically this tag shouldn't have an excerpt because it's not meant to stay around. It gets automatically deleted like any other tag when there are no questions with it - which it shouldn't have questions with it - and that deletion will clear out the wiki and excerpt with it as well, resetting it to blank when it gets used in the future (if ever).
Only sites that have historical questions that legitimately don't fit into any site tags anymore leave questions with the tag, and the description is created to indicate why. But please do not bother adding this information to the tag on other sites. It is a complete waste of your and reviewers' time. Your time is far better spent retagging the questions so they're not using this tag in the first place.
