Last modified: 2013-07-19 02:56:17 UTC
I saw it once on https://zh.wikipedia.org/wiki/%E4%BA%94%E9%A9%AC%E8%A1%97%E9%81%93 and it disappears on action=purge. Dunno why.
It looks like the page would've been cached when the last edit was made. If I checkout a revision made on the date of the last edit to that page (not quite the one in use on zhwiki at the time but it should be about right), create a page with __DISAMBIG__ and save it, I see a raw "__DISAMBIG__". If I then get the latest master, enable the Disambiguator extension, and purge the page it disappears. (This happens even without the extension) Therefore this seems to be because of old parser behaviour, and nothing to do with the Disambiguator extension.
> If I checkout a revision made on the date of the last edit to that page I should probably make it clear that this was a core git commit, 6ce38ed838c88918233c45ab6d8ac84a1520a290 specifically.
There was a report of this in #wikimedia-tech on one of the French wikis yesterday, I believe.