Steve and Mary-Ann. Originally suggested by User:Godboy in his fanfic "Another night in the life of Waldo and Steve", this is one of the most evil pairings ever made. Sometimes abbreviated as S&M, which has some extra connotations, I think? See also: X&Y, Steve and Waldo.
This idea also originated SMART, the society of Steve And Mary-Ann's Repulsive Twits, many of them would love to see S&M-A made canon in CRFH. See Board Societies and Coalitions.