Auto-Correct will only work on the most common spelling errors. Auto-Correct doesn't always know what word you mean. Words like write, rite, right and Orr, ore, or cannot be corrected without knowing other words in the sentence. Outside of word processors that will check the use within the sentence, simple spell checkers will not and just flag the word for you to pick from a selection.
I've had Auto-Correct bite me a few times so it's not good to rely on it 100%.