When your someone page URL is missing so it cause 404 not found errors.
In happens because sometimes when you change your old pages URLs but till that time your old URLs had been indexed to Google. That is why still traffic coming on those pages and it causing 404 not found error.
Sometimes if you removed some pages, but that old link presence still exist so you need to remove it by follwoing ways.
* Remove the links presence manually where you placed in past.
* Request to Google to remove your URLs by using GWT (Google Webmaster Tools).