Recently, I came to knew that if any user during the contest takes the code of his/her friend and submits it then the user is skipped for the contest (without any penalty) while his friend gets notification.
Let us say there is a person X, and he is not performing well in the contest (so his/her rating will fall). What he does is create a temporary codeforces id, submits code for question A in fake id. Then X again submits the code for A. So, X is out of contest without any penalty. Can this be done? and if it can be, then this is a loop hole i think.