Google Webmaster Tools seems to be generating a lot of fake errors. About a month ago they fixed the false 404 errors and now this month, they are reporting false HTTP errors. Here is a screen capture of the overview report from my Webmaster Tools login area for this site:

There are a dozen or so reports on this issue at Google Groups with others complaining about this issue. It worries webmasters to see this issue. For me, I just brushed it off, but I can see how it can really worry e-commerce sites and publishers.

JohnMu of Google has confirmed the issue, calling it just a reporting glitch that will be fixed shortly. He said:

True, the feedback in the groups is an invaluable tool for Google to improve and fix issues.

Forum discussion at Google Groups.