We lost traffic on a site recently and after digging for reasons, found a change in Google AND Bing WebmasterTools related to site indexing. (blocked a bunch of urls) In this case, I think it was the updating of a plugin, which caused (unknown) changes to the robots.txt, which resulted in a large number of files being put into the index, then excluded in robots.txt. Either way... not good.
The site has been on autopilot for so long, I had not thought about checking robots.txt or the sitemap.xml until the traffic fluctuation.
Anyhow - I am trying to find a plugin that will track 100% of EVERY change that occurs on a self-hosted WordPress system, any ideas?