I noticed what I would consider odd behaviour on the wiki this morning. For some reason I was logged out, but hadn't noticed; I went to Category:Categories and panicked as many of the categories there had disappeared - for instance, I was looking for Category:Creatures. I totally understand why someone logged in would be denied editing access and things like that, but why would they be denied access to a full listing of a particular set of categories? Many casual readers of the wiki, who just don't log in, may get this problem and never know the full value of the wiki. It smells like a bug to me.
I double checked it by logging in and out again and got consistent behaviour:
Thanks to Cpt kirstov for pointing this thread out to me. It me a bit to figure out what's going on, but it's realted to how we do caching for anonymous users. Anons actually get a cached copy if one exists and it is current. Pages caches get marked for regeneration when pages are edited. In most cases, this isn't a problem... pages get edited, and a fresh cache is generated the next time an anon requests the page. The problem with Category pages is that they aren't actually edited all that often. Instead we add categories to pages, then some magic happens and the category page gets updated dynamically. The problem is that the file cache doesn't get cleared this way.
There are two solutions to this problem. First, I should find a way to script a periodic cache purge for Categories. Second, anybody can manually purge the cache of a specific page by adding ?action=purge to the end of the pages url.
For the record, this should now be mostly fixed. Category caches now get regenerated every night (specifically, 0700 GMT). Also, this same issues was affecting the front page... folks not logged in were not seeing the newest featured articles (this is also fixed every Monday at 0030 GMT). If anybody notices that anon users are still getting old versions of the page after the caches are regenerated, please let me know.
Hmm, got randomly logged out again this morning while on Category:Planes page and saw this error once more. Logged in I see 30 subcategories and 39 pages. Logged out I see 6 and 36, respectively, but that includes some old subcategories I deleted some days ago. It's consistent logging in and out again plus a Ctrl-Shift-R in Firefox makes no difference. The original issues with Category:Creatures continues to be solved: I checked. Could this be a timing issue: a logged out user sees the page as it was some days previously?