One characteristic of idioms is their fixity, which is to say that they are not subject to alteration at the whim of the speaker/writer. When a radio announcer with good diction who is otherwise articulate says “cut him a break” (as did Steve Inskeep, “Morning Edition,” NPR, December 20, 2010) instead of the idiomatic “give him a break,” one can easily trace the source of the mistake (“cut him some slack”) and recognize it as an instance of contamination. All the same, it is a catachresis nonetheless, stylistically offensive and bordering on the ungrammatical.