I see this mentioned a few times on Googling it but I don't think it happens very often and I doubt it's a SL application page bug. Clearing internet browsing history (i.e temporary files, history, cookies, saved passwords and web form info) before trying again to enter password may very well fix it.
If that does not work, and the person still exeriences that password issue trying to join Second Life, all he/she has to do is go to the Second Life Home page, click the Help menu, log in with a guest email and from the Support menu select Account Creation Issues. Support should be able to assist.