Since November 14th, I've been seeing a large uptick in complaints about Google Webmaster Tools reporting DNS issues with sites, leading to Google not being able to index sites fully.
In April, Google had a bug where they sent out false DNS errors to many webmasters and told everyone to ignore them. I am not sure if this is an issue with Google or something else.
The strange part is that it coincides with reports of ranking changes that I don't see myself.
I am going to try to get word from Google about the DNS issue and see if it was a bug and/or if it may be in any way related to the changes some tools noticed in regards to ranking fluctuations on November 14th.
Update: Google told me, "we're not seeing anything unusual on our end." :-)