aakk9999 - 8:01 pm on Oct 12, 2012 (gmt 0)
Mods will soon remove URL in your post - you should have used example.com.
And yes, mods have removed URL whilst I was typing the response...
Anyway, with regards to your question:
Firstly, it would be good if you crawl your site with a tool such as Xenu's Link Sleuth and then inspect the crawl results to confirm that there is indeed no such link on your site.
It is also possible that link existed in the past and that you fixed it since, but Google is still requesting it. If Google has not re-crawled the page that had incorrect link (the page that you fixed), it will think the link is still there out on the web and from my experience these links get crawled more often and reported as errors in WMT.
And then it is also possible that someone linked to that page whilst the link was not in correct format and therefore such link exist somewhere else on the web - and Google will be re-trying it and reporting the error.
Once you fix the error (which appears you have), and the fixed page has been re-crawled by google, then I would declare WMT error "Fixed". From my experience, once the page that returns 404 is not linked from anywhere, and you cleared error in WMT via "Fixed", then the error for this URL will not appear again in WMT error report (this may take a while and may need a several cycles of declaring URL "Fixed"). But eventually Gooogle will drop this error from WMT errors report.
It is also worth knowing that if there is another page (on your domain or on external domain) that links to your page using that old incorrect URL format, then the 404 error will re-appear in WMT even if you declare it as "Fixed" (in which case you will probably see which page links to it, i.e. where did Google found it).
However, if the error reported in WMT is 404, I would not worry as long as your site is not linking to such URL. The 404 report in WMT is useful exactly for this reason - to check whether your own site is inadvertly linking to incorrect URL format. Otherwise, just ignore the error.