I quickly discovered that I should re-frame the question: What are the most difficult, commonly used words in the English language?
There are plenty of obscure, impossible-to-spell words. These are words used at national spelling bees, such as stichomythia or succedaneum. However, when was the last time you used “succedaneum” in a press release?
Below is a collection of commonplace words that people misspell the most, based on a consensus from online dictionaries (Oxford Dictionaries, Merriam-Webster and Wordnik), writing websites (Purdue Online Writing Lab and Plainlanguage.gov) and PR Daily articles: