A Google Webmaster Help thread has reports of some webmasters claiming their Google Webmaster Tools impression data dropped to zero on December 28th/29th.
Why do I think this is a reporting error over a webmaster error?
Gary Illyes from Google responded in a way that made me believe it was a bug with Google's reporting engine:
I let the engineers know about the problem with www.zintro.com and they're going to take a look.
Webmaster Stuart said:
On December 28 2012 my site zintro.com dropped impressions, clicks and traffic to almost 0 in a matter of minutes and has not gone back up. Everything in my server is ok, and the webite has not had any major changes. If you search zintro on google it doesnt even show up until the second page, the same happens with zintro.com.
There are no errors or messages on webmaster tools and I really don't understand what happened.
Please help me, I am very frustrated with this problem and it doesn't appear to have a fix on our side.
Others have seen the same issue.
Forum discussion at Google Webmaster Help.