Spelling
I watched a presentation which was otherwise quite good, the presenter had also dressed corporate, was a fairly good speaker, but the presentation was grouped under headings of 1st principle, 2nd principle etc. except every time it was spelled "principal." I could not help being distracted, waiting for the next principal to trot on screen, nor thinking the speaker was less intelligent, perhaps not stupid but definitely I would have rated him higher in my mind if he had spelled the word correctly. I don't see spelling as superfluous, nor grammar even if there are grammar nazis that do pick on poor split infinitives like it was an actual rule.
In case of the said principle vs principal the meaning of the word is changed, obscuring the meaning the speaker tries to get across, so it does interfere with communication. There are other common ones like their / there, your / you're and while it may be obvious from the context it at least makes it slower for reader to get to the meaning. And let's face it, would you trust your case to a lawyer who advertises "Had an accident that was not you're fault?"
