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I can't tell you how many times I've tried logging into HN using the signup form (which, of course, doesn't ask for your password confirmation). The password confirmation, I think, has become a necessary input field to let users instantly recognize and know that they're signing up for an account. Qwerty keyboards, for example, aren't the most efficient way to type, but the layout has become so ingrained into our brains that we can't imagine any other way to type.