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:
Mouseover of each URL makes an immediate call to the site from IP 74.125.nnn.nnn with same or different IPs for each request, using user agent:Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/536.8 (KHTML, like Gecko; Google Web Preview) Chrome/19.0.1084.36 Safari/536.8.
The referrer is http://www.google.com/search each time.
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.
