If you login to Google Webmaster Tools, then go to the crawl errors reports and place your mouse over one of the URLs listed in the report, a overlay will pop up with a preview.

This seems to be a new feature that I didn’t notice being announced by Google yet. It seems to work best with soft 404s because often, real 404s do not have any real landing page (although they should).

Here is a screen shot:

Last week, Google added alerts for crawl errors but made no mention of this feature. Google is, however, constantly tweaking Google features on Google Webmaster Tools.

A WebmasterWorld thread is having discussion around this now. A user said:

Would this be useful for you? Maybe if Google showed you the specific part of the page with the error but with 404s, I am not sure if it makes sense?

Forum discussion at WebmasterWorld.

Update: These preview links are now on all links within the tool.