g1smd - 1:56 am on Feb 17, 2012 (gmt 0)
Web Preview is a bit flaky to say the least and the tools in WMT are quite unreliable.
In WMT the "Pre-render Desktop Search Instant Preview" image is completely broken for every page on a site I am currently looking at. There's a message below the images to say that there were many errors fetching resources from the site. However, looking in the server logs for those resources shows that Google requested all of them and all were served in full and with "200 OK" status. Requests came from multiple IPs to grab the page, images, scripts, etc in a very short time.
For those files tagged as "Fetch failure" all I can assume is that Google's rendering script didn't wait long enough for the fetch part of the process to finish fetching them all.