To be fair, Roberts actually messed up as well: Roberts said “. . . I will execute the Office of President of the United States faithfully . . .” The oath is supposed to be “. . . I will faithfully execute the Office of President of the United States . . .”
Bush appointed Roberts, thus screwing up the oath is:
“Bush’s Fault”
Roberts is probably a grammar nut ... the adverb is supposed to come after the verb. He said it properly, just not the way it was written.