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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.
Comments:
Praveen Sharma
07/17/2012 12:47 pm
I noticed it in the morning, but don't think its something which is advantageous to webmasters. May be different webmasters would have different views.
Qudos
07/17/2012 01:44 pm
Thanks for update.
David Portney
07/17/2012 03:54 pm
I agree with you Praveen, this gets a yawn out of me.
Praveen Sharma
07/17/2012 05:10 pm
Exactly David, I don't see any point in having this mouse-over preview. Its one of the bizarre updates from Google.
SEO Generation
07/18/2012 02:59 pm
If there are all 404s, then it defeats the whole purpose of being a preview. Ideally you want to preview live pages ...
Richard Gailey
07/18/2012 11:26 pm
I noticed this yesterday, but honestly didn't think anything of it as I figured I had only just realised it was there. Incidentally, WordPress have been doing this for a while. However, they have been doing it on the section where you check for your spam via Askimet. If you hover your mouse over a website that they have included in their profile, it gives you a decent idea to see whether it's a spammy site or now.