And on that note, let's give a big hand to forms that automatically tab you into the username field about five seconds after the page loads thanks to shitty JavaScript, so if you breeze through login too fast without looking up, you end up trying to log in as "stevethepocketssword12345", and then that's stuck as one of your autocompletes unless you want to erase all your browser data.
If you disable cookies, I think it removes that list. But it would come at a cost- everytime you try to log into your gmail, it would send you a mail about your computer being unfamiliar, and you would have to make some sort of spam filter for google auto-mails.