Maybe there is a way to do this that I'm missing without editing the .jsp file.
The login CAPTCHA needs autocomplete="off" so when prompted and starting to type in a response, a list of suggestions using previous CAPTCHA's doesn't cover the CAPTCHA image.
Pretty simple change. I don't like editing core, but it's simple enough to get rid of the annoyance. There's no reason a CAPTCHA should have an autocomplete.