He did ask if you were referring to
Mail Fetcher (which is what you use for your Gmail retrieve e-mail from non-Gmail accounts via POP3), or if you were referring to an e-mail client (ie., Thunderbird, MS Outlook) trying to fetch Gmail via POP3.
That in turn is also different from people on non Gmail servers
forwarding e-mail to a Gmail account, which you briefly touched on as well. That doesn't have to do with POP3/IMAP, but SMTP instead.
So... AFAIK, Mail Fetcher will complain the credentials are incorrect when it cannot login to the external POP3/IMAP account, and will not hammer the server indefinitely with the incorrect credentials.
Easiest way to be absolutely certain is to set yourself up with a Gmail account, add a Mail Fetcher with the correct credentials, and then change the credentials on the server (so Gmail now has the incorrect credentials)