Why can't I play certain Call of Duty online servers?

When I'm playing Call of Duty Multiplayer on the computer, this error thing pops up and says:





Unpure client detected. Invalid .PK3


File Reference


C:\PROGRA~1\CALLOF~1\SFMMOD1\ZzZ_MODPa...





I know its a SFM Mod, but why can't I play it?

Why can't I play certain Call of Duty online servers?
ignor the question above if someone has the same product key as u then u wuldnt be able to install the game. everyone has different product keys. ur problem is. is your firewall. and if its not that then its your modems firewall.. u may need to fiddle with your modem.
Reply:You can't play in some servers probably because you have the same product key as someone in the server. I don't know much about Call of Duty, but i do play halo (and in halo you can't play in a server if someone in the server has your product key).

redbud