I don't mean to gripe, but why is this interesting? The HR staff at a startup made a common and mildly embarrassing mistake that is unlikely to be of any long-term significance.
I'll be the last to say that was stupid, not that long ago I made a similar gaffe. Ever since I have the autocompletion in my email address bar set to off, and I would advice everybody that deals with sensitve data on a regular basis to do the same. It's all too easy to send an email to entirely the wrong person like that.
Oh, and another easily avoided error is 'reply to all'.
Twitter in now under the microscope of the public and media so they have to take extra steps to preserve their image. Of all their departments, HR should be the one that best represents their brand/company and this oversight is unprofessional and hopefully not indicative of a problem with their corporate culture.