Boneprone, that is because no mater hjow good you are with a site you will still get 404's
The reason being if you get linked to alot some times ppl put the wrong links up back to you thus creating 404 traffic......
Also if you have had a domain for a long period of time 404's are naturaly generated because of old files that once were there but now are gone or by simply moving hosts you can loose files n shit which would generate 404 traffic......
Also One key thing is htaccess you can prevent hotlinkers and other cool stuff and send the bastards hotlinking traffic directly to your sponser as well

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