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I use a password manager (LastPass) that always types my password 100% correctly for me and I’ve never had issues like these.
Maybe you’re typing too fast and accidentally skipping a keystroke here and there? Or maybe you start typing before the page is fully loaded and it doesn’t register your password fully.
Like [USER=43147]@Gizmo[/USER] said, coding something that would recognise a correct password as incorrect seems to go against UX principles.