If it offers up "dallas" for "dal", but the user keeps typing, then it's reasonable to assume "dallas" wasn't what they were looking for, or they'd have picked it after "dal".
Exactly. I'm not typing D...pause...A...pause...L wait for suggested value. I'm typing dall by the time the first suggestion has had a chance to make its appearance. Then again, I'm on a real computer with a keyboard, and not some mobile device where it's impossible to type properly and forces those pauses between letters where the autocomplete might be noticeable.