For example, your site may use the following code to track file downloads in Google Analytics:
<a href="helloworld.pdf" onClick="_gaq.push(['_trackPageview','/download-helloworld']);">Hello World PDF</a>
When it sees this, as an example, Googlebot may try to crawl the URL http://www.example.com/download-helloworld, even though it’s not a real page. In this case, the link may appear as a 404 (Not Found) error in the Crawl Errors feature in Webmaster Tools.
Google strives to detect these types of issues and resolve them so that they will disappear from Crawl Errors. In general, 404 errors won’t impact your site’s search performance, and you can safely ignore them if you’re certain that the URLs should not exist on your site. It’s important to make sure that these and other invalid URLs return a proper 404 HTTP response code, and that they are not blocked by the site’s robots.txt file.