this has been happening to me when using google instant and really annoying me. if i type slowly, it wont correct on me, if i type fast it does correction. sometimes large ones.
Unrelated to autocorrection, but my little rant. If I type fast Google instant only catches the first half of what I type. I don't know what kind of javascript runs instant but it fails for fast typers. Happens on different computers and different browsers. =(