You'll learn a simple truth early in TFBW: Cartman will never leave you alone. Consequently, there are plenty of Easter eggs involving Cartman, and this one is no different. As you progress, you'll come across a secret code to Cartman's basement, one that has to be entered on a numberpad in order to unlock the doors found there.
If you cheat (as most people do) and find the code online ("307"), you can just try entering it on the numberpad when you first come across it. However, the game will know if you cheated, and Cartman's avatar will appear on the screen dressed in a New England Patriots' jersey, shaming you for cheating and mirroring Tom Brady's deflate-gate controversy.