Example from a friend-who-shall-remain-anonymous: paradigm, but pronounced "paradiggum."
Or alternatively, you hear a word in conversation and understand it, but mangle it when trying to spell it, or don't recognize it when it's written.
Example from, um, me: segue. Until last year, I honestly thought it was spelled how it's pronounced, "segway." What's more, I thought that when the word segue appeared in sheet music, it was pronouced "seeg."