The category is added dynamically by a tag extension. Any page that
uses the tag <foobarblat> is automatically placed into category "Foo".
Doing it at article-save time would not work, since the tag could be
present by transclusion (e.g., added later to a template used by the
article).
We use this technique to keep track of all articles that use some
resource-intensive tags. (For example, accessing external databases.)
DanB
-----Original Message-----
Thomas Dalton asks:
When are you adding this category? Can you not just concatenate the
wikitext onto the end of the article when it's saved?