A word that I perpetually misspell is "impossible". I want to add an extra "i" for whatever reason, making it "impossibile." I don't know when or why I started doing this, but it's one of my most backspaced words.
Slightly complicating things slightly is that "impossibile" is the correct spelling the same word in Italian, which is not a language I speak (sadly) but is a language that auto-checks that look for different languages are occasionally good at picking up.