Lion gained a spelling correction feature from iOS that many people find… annoying, to be polite about it. Now, OS X has long underlined misspelled words in red while you write in almost any application. But taking a cue from its lil’ i-brother, Lion will autocorrect some words and replace the red underline with blue to highlight what it’s done.
If you’re not happy with Lion’s choice—perhaps you meant to misspell a word or you’re simply typing a word that Oxford doesn’t recognize yet—there’s no need to right-click or delete the word and try again. Simply hit ⌘-Z right after the blue underline appears to reverse OS X’s changes and keep on typing.