There are a lot of little phrases and words that people only hear, never read, and so they don't know how to spell them or even what words are in them. There are some that are slang terms that almost everyone only hears, and then there are some that only get misspelled by those who don't read much -- like the title of this post, one of my favorites. I like to collect these. (Yes, I collect words; they're cheaper than stamps. Someday I'll talk about fossil phrases here, too.)
How about "beaucoup" (which is usually spelled "buko")? One that I don't know a proper spelling for, if there is one, is "bupkis". I know it is frequently argued whether the proper phrasing is "you've got another think coming" or "you've got another thing coming" (I lean towards the latter). Another one I see a lot is "persay".
Suggest a few others in comments to this post.