Did Chief Justice Roberts mess up the Constitutional oath?  Or was he just a stickler for proper English grammar?

The oath says "…will faithfully execute the Office of the President of the United States…".  Roberts prompted Obama to say "…to execute the Office of the President of the United States faithfully…"

But this op-ed suggests that Roberts simply was using proper English by not splitting his verbs:

But a simpler explanation is that the wayward adverb in the passage is blowback from Chief Justice Roberts’s habit of grammatical niggling.

Language pedants hew to an oral tradition of shibboleths that have no basis in logic or style, that have been defied by great writers for centuries, and that have been disavowed by every thoughtful usage manual. Nonetheless, they refuse to go away, perpetuated by the Gotcha! Gang and meekly obeyed by insecure writers.

This doesn't strike me as plausible.  The oath of office – yes, with the split infinitive — is set out in the constitution:

"I do solemnly swear (or affirm) that I will faithfully execute the office of President of the United States, and will to the best of my ability, preserve, protect and defend the Constitution of the United States."

Roberts is a strict textualist.  There is no way he would veer from the actual text of the Constitution in order to make a grammatical (and arcane) point. 

Furthermore, very quick research has shown that Roberts, in his legal opinions, is no stranger to split-verb usage, as well as infinitive-splitting.

It should be also be noted that "to the best of my ability" also splits some verbs.  And Roberts didn't screw around with that.

So the final verdict?  It was a flub.