Calvin does it himself in another strip. After hurting Susie's feelings by calling her names, he makes a hesitant attempt at apology. Irritated and not understanding his stammering, Susie snaps at him and stalks off. Calvin shouts "I'm trying to apologize, you dumb noodleloaf!" Realizing he just called her a name while trying to apologize for calling her names, he facepalms.
There's a strip where Garfield is facepalming for no reason. Jon asks him what he's doing, and Garfield replies, "Practicing being exasperated." Jon notices he's not wearing pants, and Garfield puts his practice to good use.