Yesterday we discovered google was indexing banner ads as the blurb below search results, instead of actual wiki content. This was caused partly by a change in the banner URL, and the fact that we did not update robots.txt for the new URL.

Although the content of http://en.wikipedia.org/robots.txt is fixed, the Last Modified header is still 11/2013 -- date of the last wiki update. Details from Brad Jorsch:

That's the timestamp of the last edit to enwiki's MediaWiki:Robots.txt, see
https://en.wikipedia.org/w/index.php?title=MediaWiki:Robots.txt&action=history
[en.wikipedia.org] (and also w/robots.php in the operations/mediawiki-config repo).
 
Probably we should use the max of that or /srv/mediawiki/robots.txt to really get the
last-modified timestamp.

As a temporary workaround we're going to do a no-op wiki edit to bump that timestamp.