I have a lot of 404 errors on my webmaster's page. I don't know how google ended up on those urls. I checked my site many times to see if any links to those pages appear, but found nowhere. Is google trying out new urls on its own depending on the url structure?. Will these 404 errors result in bad outcome to my seo.
More than likly these URLs are a result of other sites linking to your site, perhaps a site that used to exist on the domain. Google keeps looking for them because the other site thinks the links are still valid.
The best way to tell Google these pages are gone forever is to return the 410 gone status when the robot visits.
In your .htaccess file use the following code:-
#Stuff to 410 Redirect gone /path/to/page.html Redirect gone /directory-path/