Even the best spellers are tripped up from time to time, either through inattentiveness (not catching a typo) or unawareness (following logic and spelling sacrilegious as sacreligous). Not-bad spellers may have replaced the final a of ad nauseam with a u for a lifetime but never noticed because the misspelled version appears so regularly in print and on TV. Poor spellers may rely on their computerized spell checker, but reliable it is not: “I shed a tier  or two as I poured over the tale of the urchin and the hurtles she faced” will breeze through a spellcheck “wall” without a scratch.

Before spelling was standardized in the eighteenth and nineteenth centuries, Shakespeare and his peers could spell the same word pretty much as they saw fit. Today you could too, but you do so at your peril when a misspelling really matters – e.g., in a job application letter,  a term paper, or a thank-you note to your  retired English-teacher aunt.