Good guess. Since the web came to be, that's probably the #1 misspelling that bugs a lot of readers, pardon the pun. Spelling is lost art. There's no ' a ' in definitely though it might be pronounced as if it did have an ' a '.  English is not a true phonetic language, so spelling by sound doesn't work well. (Try to say "delight" as spelled. lol)  Studying roots is key to spelling. For example, root of "definitely" is Latin, finis, which means end, limit, or boundary. Finite, infinity, definite, define, refine, confine, ad infinitum.

Most commonly misspelled word in English is parallel.